[1] An application layer abstraction is specified in both the Internet Protocol Suite (TCP/IP) and the OSI model.
[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.
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] Originally the OSI model consisted of two kinds of application layer services with their related protocols.