Application layer

An application layer is an abstraction layer that specifies the shared communications protocols and interface methods used by hosts in a communications network.[1] The application layer abstraction is used in both of the standard models of computer networking: the Internet Protocol Suite (TCP/IP) and the OSI model.[2] Although both models use the same term for their respective highest level layer, the detailed definitions and purposes are different.[3]

TCP/IP

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.[4] 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.[5] 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.[6][7]

OSI model

In the OSI model, the definition of the application layer is narrower in scope.[8] The OSI model defines the application layer as the user interface responsible for displaying received information to the user.[9] In contrast, the Internet Protocol Suite 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.[9]

Internet protocol suite
Application layer
Transport layer
Internet layer
Link layer
  • more...
OSI model
by layer

Protocols

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 the functionality of the early Internet:[6]

Examples
  • DNS, Domain Name Services
gollark: It might also be useful to look into moving some common stuff like fetch, fread/fwrite and all that into a big library...
gollark: Oh, come to think of it, it would be cool if potatOS could do P2P update if there's no internet connection somehow. Which is probably one of the things git is designed for. Hmmm.
gollark: I have backups of various older versions of it, too.
gollark: No, there are just a lot of files on pastebin and it's hard to track down all the places potatOS randomly downloads things.
gollark: ... if I can find it, actually.

References
  1. "Application Layer | Layer 7". The OSI-Model. Retrieved November 5, 2019.
  2. "Four Layers of TCP/IP model, Comparison and Difference between TCP/IP and OSI models". www.omnisecu.com. Retrieved November 5, 2019.
  3. "What is the difference between TCP/IP and IP protocol?". SearchNetworking. Retrieved November 5, 2019.
  4. "What is the difference between TCP/IP and IP protocol?". SearchNetworking. Retrieved November 5, 2019.
  5. SEO, Bradley Mitchell An MIT graduate who brings years of technical experience to articles on; computers; Networking, Wireless. "What Is Transmission Control Protocol/Internet Protocol (TCP/IP)?". Lifewire. Retrieved November 5, 2019.
  6. Robert Braden, ed. (October 1989). "RFC 1123: Requirements for Internet Hosts – Application and Support". Network Working Group of the IETF.
  7. "What is the Application Layer?". www.tutorialspoint.com. Retrieved November 5, 2019.
  8. Shaw, Keith (October 22, 2018). "The OSI model explained: How to understand (and remember) the 7 layer network model". Network World. Retrieved November 5, 2019.
  9. "What Is The OSI Model?". CloudFlare. 2019. Retrieved November 4, 2019.
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