The application layer is an abstraction layer reserved for 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 to establish host-to-host connections.
In the OSI model, the definition of its application layer is narrower in scope. The OSI model defines the application layer as being the user interface. The OSI application layer is responsible for displaying data and images to the user in a human-recognizable format and to interface with the presentation layer below it.
It separates functionality 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.
TCP/IP protocols
The following protocols are explicitly mentioned in RFC 1123 (1989), describing the application layer of the Internet protocol suite.
- Remote login category
- Telnet
- File transfer category
- FTP
- TFTP
- Electronic mail category
- SMTP
- IMAP
- POP
- Support services category
- DNS
- RARP
- BOOTP
- SNMP
- CMOT
Other protocol examples
- 9P, Plan 9 from Bell Labs distributed file system protocol
- AFP,
- APPC, Advanced Program-to-Program Communication
- AMQP, Advanced Message Queuing Protocol
- BitTorrent
- Atom Publishing Protocol
- CFDP, Coherent File Distribution Protocol
- CoAP, Constrained Application Protocol
- DDS, Data Distribution Service
- DeviceNet
- eDonkey ETC.,
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