An application layer is an
abstraction layer that specifies the shared
communication protocols and
interface methods used by
hosts in a communications network.[1] An application layer abstraction is specified in both the
Internet Protocol Suite (TCP/IP) and the
OSI model.[2] Although both models use the same term for their respective highest-level layer, the detailed definitions and purposes are different.[3]
In the Internet protocol suite, the application layer contains the communications protocols and interface methods used in process-to-process communications across an
Internet Protocol (IP) computer network.[4] The application layer only standardizes communication and depends upon the underlying
transport layer protocols to establish host-to-host data transfer channels and manage the data exchange in a
client–server or
peer-to-peer networking model.[5] Though the TCP/IP application layer does not describe specific rules or data formats that applications must consider when communicating, the original specification (in
RFC1123) does rely on and recommend the
robustness principle for application design.[6][7]
In the
OSI model, the definition of the application layer is narrower in scope.[9] The OSI model defines the application layer as only the interface responsible for communicating with host-based and user-facing applications.[10] OSI then explicitly distinguishes the functionality of two additional layers, the
session layer and
presentation layer, as separate levels below the application layer and above the transport layer. OSI specifies a strict modular separation of functionality at these layers and provides
protocol implementations for each. In contrast, the Internet Protocol Suite compiles these functions into a single layer.[10]
Sublayers
Originally the OSI model consisted of two kinds of application layer services with their related protocols.[11] These two sublayers are the common application service element (CASE) and specific application service element (SASE).[12] Generally, an application layer protocol is realized by the use of the functionality of a number of application service elements.[13] Some application service elements invoke different procedures based on the version of the session service available.[14]
CASE
The common application service element sublayer provides services for the application layer and request services from the session layer.
It provides support for common application services, such as:
The IETF definition document for the application layer in the Internet Protocol Suite is RFC 1123. It provided an initial set of protocols that covered the major aspects of the functionality of the early
Internet:[6]