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In the Open Systems Interconnection (OSI) communications model, the application layer provides services for an application program to ensure that effective communication with another application program in a network is possible. The application layer is not the application itself that is doing the communication. It is a service layer that provides these services:
- Makes sure that the other party is identified and can be reached
- If appropriate, authenticates either the message sender or receiver or both
- Makes sure that necessary communication resources exist (for example, is there a modem in the sender's computer?)
- Ensures agreement at both ends about error recovery procedures, data integrity, and privacy
- Determines protocol and data syntax rules at the application level
It may be convenient to think of the application layer as the high-level set-up services for the application program or an interactive user.