Key Architectural Principles

From Wikipedia
Alan Isherwoo
Key Architectural Principles

An early architectural document, RFC 1122, emphasizes architectural principles over layering.

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 simplicity. Real-world needs for firewalls, network address translators, web content caches and the like have forced changes in this Principle. [3]
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 -- have 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 only refers to the existence of the "internetworking layer" and generally to "upper layers"; this document was intended as a 1996 "snapshot" of the architecture: "The Internet and its architecture have grown in evolutionary fashion from modest beginnings, rather than from a Grand Plan. While this process of evolution is one of the main reasons for the technology's success, it nevertheless seems useful to record a snapshot of the current principles of the Internet architecture."

No document officially specifies the model, another reason to deemphasize the emphasis on layering. Different names are given to the layers by different documents, and different numbers of layers are shown by different documents.

There are versions of this model with four layers and with five [citation needed] layers. RFC 1122 on Host Requirements makes general reference to layering, but refers to many other architectural principles not emphasizing layering. It loosely defines a four-layer version, with the layers having names, not numbers, as

Process Layer or Application Layer: this is where the "higher level" protocols such as SMTP, FTP, SSH, HTTP, etc. operate.
Host-To-Host (Transport) Layer: this is where flow-control and connection protocols exist, such as TCP. This layer deals with opening and maintaining connections, ensuring that packets are in fact received.
Internet or Internetworking Layer: this layer defines IP addresses, with many routing schemes for navigating packets from one IP address to another.
Network Access Layer: this layer describes both the protocols (i.e., the OSI Data Link Layer) used to mediate access to shared media, and the physical protocols and technologies necessary for communications from individual hosts to a medium.
The Internet protocol suite (and corresponding protocol stack), and its layering model, were in use before the OSI model was established. Since then, the TCP/IP model has been compared with the OSI model numerous times in books and classrooms. In a popular textbook, which is a secondary source,[4] but not primary IETF specifications, the model has evolved into a five-layer version that splits the network access layer into a Physical layer and a Network Access layer, corresponding to the physical layer and data link layer of the OSI model. The Internet or Internetworking layer corresponds to the OSI Network layer