The Application layer is responsible for directly accessing the underlying processes that manage and deliver communication to the human network. This layer serves as the source and destination of communications across data networks.
The Application layer applications, protocols, and services enable users to interact with the data network in a way that is meaningful and effective.
Applications are computer programs with which the user interacts and which initiate the data transfer process at the user's request.
Services are background programs that provide the connection between the Application layer and the lower layers of the networking model.
Protocols provide a structure of agreed-upon rules and processes that ensure services running on one particular device can send and receive data from a range of different network devices.
Delivery of data over the network can be requested from a server by a client, or between devices that operate in a peer-to-peer arrangement, where the client/server relationship is established according to which device is the source and destination at that time. Messages are exchanged between the Application layer services at each end device in accordance with the protocol specifications to establish and use these relationships.
Protocols like HTTP, for example, support the delivery of web pages to end devices. SMTP/POP protocols support sending and receiving e-mail. SMB enables users to share files. DNS resolves the human legible names used to refer to network resources into numeric addresses usable by the network.
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