application layer

Definition of application layer in The Network Encyclopedia.

What is Application Layer?

Layer 7 of the Open Systems Interconnection (OSI) reference model, in which network-aware, user-controlled software is implemented—for example, e-mail, file transfer utilities, and terminal access.

The application layer represents the window between the user and the network. Examples of protocols that run at the application layer include File Transfer Protocol (FTP), Hypertext Transfer Protocol (HTTP), telnet, and similar protocols that can be implemented as utilities the user can interface with.

Originally the OSI model consisted of two kinds of application layer services with their related protocols:

  • Common Application Service Elements (CASE):
    These enable users to initiate or terminate peer (Layer 7 to Layer 7) connections between different machines on the network, enable reliable transfer of information between them, initiate remote operations, and support recovery from failure and rollback steps.


  • Specific Application Service Elements (SASE):
    These can include file transfer, message handling, terminal access, and other services that make use of the CASE elements and protocols.


These terms have largely been replaced with the term Application Service Elements (ASE) to describe the elements of the application layer.

In most real-world networking, such as TCP/IP networking, many of these application layer services have no meaning.
The layer application from the OSI model is the higher layer.