Application layer

An application layer is an abstraction layer that specifies the shared communication protocols and interface methods used by hosts in a communications network. The term is used in both the Internet protocol suite and the OSI model, though the two models define their highest-level layer differently. In the Internet protocol suite it covers process-to-process communication across an IP network and relies on the transport layer to carry data between hosts, with RFC 1123 setting out an initial set of protocols such as HTTP, FTP, SMTP, DNS and SNMP. In the OSI model its scope is narrower, because the session and presentation layers sit separately beneath it, and it was originally divided into the common and specific application service element sublayers.