■ Transport layer: Concerned with how data is moved from point A to point B. Protocols in this layer can be TCP or User Datagram Protocol (UDP); it also deals with the integrity and retransmission of data in the event of a failure.
■ Session layer: Concerned with making, you guessed it, a session between two machines, to be ready for sending data that is passed to it by upper layers using the lower layers to transport this data to its destination.
■ Presentation layer: Concerned with how data is represented. For example, HTML, JPEG, or MP3 formats would all reside here.
■ Application layer: Concerned with applications that use the network protocol. Applications can be SMTP, Hypertext Transport Protocol (HTTP), and FTP.
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