Application Layer

he application layer is used by most programs for network communication. Data is passed from the program in an application-specific format, then encapsulated into a transport layer protocol.

Since the IP stack has no layers between the application and transport layers, the application layer must include any protocols that act like the OSI's presentation and session layer protocols. This is usually done through libraries.

Data sent over the network is passed into the application layer where it is encapsulated into the application layer protocol. From there, the data is passed down into the lower layer protocol of the transport layer.

The two most common end-to-end protocols are TCP and UDP. Common servers have specific ports assigned to them (HTTP has port 80; Telnet has port 23; etc.) while clients use ephemeral ports. Some protocols, such as File Transfer Protocol and Telnet may set up a session using a well-known port, but then redirect the actual user session to ephemeral ports.

Routers and switches do not utilize this layer but bandwidth throttling applications do, as with the Resource Reservation Protocol (RSVP).