The application layer is the topmost level of the OSI model and is the only one that directly interacts with the end-user and specifies the shared protocols and interface methods used by hosts in a communications network. It acts as an intermediate between the actual software application and the users. It provides full end-user access to a variety of shared network services like mail services, directory services, file transfer, etc. It has many responsibilities, including error handling and recovery.
The application layer determines the availability and identity of communication partners for an application with data to transmit, verifies resource availability and synchronizes communication. For the communications to be successful, the application layer protocols implemented on the source and destination host must match.
This layer provides the protocols and services required by the network-aware applications to connect with the network. Network-aware applications implement the application layer protocols and can communicate directly with the lower layers of the protocol stack. E-mail clients and web browsers are examples of these types of applications. Application programs aren’t a part of the application layer. HTTP, FTP, TFTP, POP3, and SMTP are the few examples of standards and protocols used in this layer.
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