An application layer is an abstraction layer that specifies a group protocols and methods in each of the two 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 is contains the communications protocols and methods designed for process-to-process communications across an Internet Protocol (IP) computer network. Application layer protocols use the underlying transport layer protocols, which establish host-to-host data transfer paths, to exchange data between cooperating application components, such as in the client-server model. The TCP/IP application layer does not describe specific rules or data formats that applications must consider when communicating. However, the original specification in RFC 1123 relies on and recommends the robustness principle for application design.
In the OSI model, the definition of its 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 model does not concern itself with such detail. OSI explicitly distinguishing additional functionality below the application layer, but above the transport layer at two additional levels, the session layer and the presentation layer. OSI specifies strict modular separation of functionality at these layers and provides protocol implementations for each layer.
|Internet protocol suite|
The IETF definition document for the application layer in the Internet Protocol Suite is RFC 1123. It provided an initial set of protocols that were covered the major aspects of functionality of the early Internet.
- Remote login to hosts: Telnet
- File transfer: File Transfer Protocol (FTP), Trivial File Transfer Protocol (TFTP)
- Electronic mail transport: Simple Mail Transfer Protocol (SMTP)
- Networking support: Domain Name System (DNS)
- Host initialization: BOOTP
- Remote host management: Simple Network Management Protocol (SNMP), Common Management Information Protocol over TCP (CMOT)
- Robert Braden, ed. (October 1989). "RFC 1123: Requirements for Internet Hosts – Application and Support". Network Working Group of the IETF.
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