Networking - The OSI Seven-Layer Model - The Application Layer

The purpose of the Application layer is to provide services directly to the user programs that want to use the network. All of the lower layers exist to make it possible for the Application layer to offer useful capabilities such as file transfer, remote login, and electronic mail.

In particular, the Application layer is responsible for:

Presenting a set of standard network services to application programs,
Hiding the details of lower layer operation behind well defined interfaces, and
Coordinating application level options such as authentication, capabilities, and resource sharing.

As with the Presentation and Session layers, Application layer functions are implemented entirely in software. They reside in the applications and application protocols themselves rather than in dedicated hardware devices.

The Application layer is responsible for several processes, each of which is responsible for a series of methods:

Network Service Interfaces
- Providing standardized ways for programs to request services such as file transfer, message delivery, or directory lookup, and
- Isolating application writers from the details of individual transport or network protocols.
Resource Sharing and Remote Access
- Allowing users to access files, printers, and other resources across the network as though they were local, and
- Supporting remote login and remote execution so that work can be performed on distant systems.
Application Level Reliability and Control
- Providing application specific acknowledgments or retries when the underlying transport does not, and
- Enforcing application level policies such as message size limits, quotas, and concurrency restrictions.

At this point we have reached the top of the OSI seven layer model. You may now want to review the entire stack or move on to the Conclusion.

Nah, I want to skip around:

Introduction
Data Link (Layer 2)
Transport (Layer 4)
Presentation (Layer 6)
Conclusion