The application layer is the seventh level of the seven-layer OSI
model. It interfaces directly to and performs common application services for
the application processes; it also issues requests to the presentation layer.
The common application layer services provide semantic conversion between
associated application processes. Note: Examples of common application
services of general interest include the virtual file, virtual terminal, and job
transfer and manipulation protocols.
The application layer of the four layer and five layer TCP/IP models
corresponds to the application layer, the presentation layer and session layer
in the seven layer OSI model.
The application layer is the top layer in the OSI protocol stack. Applications
that provide network features reside at this layer and access underlying
communication protocols. Examples include file access and transfer over the
network, resource sharing, and print services. The OSI model specifies that
applications must provide their own layer 7 protocols. The OSI FTAM (File
Transfer Access and Management) utility and the X.400 electronic mail standard
provide services at the OSI application layer.
In the Internet world, the application layer resides directly on top of the
TCP/IP protocol stack. In this model, the presentation layer and session layer
of the OSI protocol stack are used. The application layer talks directly with
the transport layer (TCP and UDP). Common Internet applications in the
application layer include Telnet, FTP (File Transfer Protocol), NFS (Network
File System), SMTP (Simple Mail Transport Protocol), and DNS (Domain Name
System).
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