An application layer is an abstraction layer that specifies the shared protocols and interface methods used by hosts in a communications network. The application layer abstraction is used in both of the standard models of computer networking: the Internet Protocol Suite (TCP/IP) and the Open Systems Interconnection model (OSI model).
Although both models use the same term for their respective highest level layer, the detailed definitions and purposes are different.
In TCP/IP, the application layer contains the communications protocols and interface methods used in process-to-process communications across an Internet Protocol (IP) computer network. The application layer only standardizes communication and depends upon the underlying transport layer protocols to establish host-to-host data transfer channels and manage the data exchange in a client-server or peer-to-peer networking model. Though the TCP/IP application layer does not describe specific rules or data formats that applications must consider when communicating, the original specification (in RFC 1123) does rely on and recommend the robustness principle for application design.
In the OSI model, the definition of the application layer is narrower in scope. The OSI model defines the application layer as the user interface responsible for displaying received information to the user. In contrast, the Internet Protocol model does not concern itself with such detail. OSI also explicitly distinguishes additional functionality below the application layer, but above the transport layer at two additional levels: the session layer, and the presentation layer. OSI specifies a strict modular separation of functionality at these layers and provides protocol implementations for each layer.
(Session Layer under OSI model) The IETF definition document for the application layer in the Internet Protocol Suite is RFC 1123. It provided an initial set of protocols that covered the major aspects of functionality of the early Internet.
There are commonly reoccurring problems that occur in the design and implementation of communication protocols and can be addressed by patterns from several different pattern languages: Pattern Language for Application-level Communication Protocols (CommDP),Service Design Patterns,Patterns of Enterprise Application Architecture, and Pattern-Oriented Software Architecture: A Pattern Language for Distributed Computing. The first of these pattern languages focuses on the design of protocols and not their implementations. The others address issues in either both areas or just the latter.
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