Application layer

Application layer

1. Applications such as web browsers, online games, chat and email with friends allow us to send and receive information with relative ease. In general, we can access these applications and use them without knowing how they work. However, for network professionals, it is important to know how an application can format, transmit and interpret messages that are sent and received through the network.

2. The application layer is the closest to the end user. As shown in the figure, it is the layer that provides the interface between the applications used for communication and the underlying network in which the messages are transmitted. Application layer protocols are used to exchange data between programs that run on the source and destination hosts. The top three layers of the OSI model (application, presentation and session) define functions of the single TCP / IP application layer.

3. Layers of presentation and session

   • Presentation layer
     • The presentation layer has three main functions:
       • Format the data of the source device, or present it, in a compatible manner so that the destination device receives it.
       • Compress the data in such a way that the target device can decompress it.
       • Encrypt the data for transmission and decrypt it upon receipt.
   • Session layer
     • As the name implies, the session layer functions create and maintain dialogs between the source and destination applications. The session layer handles the exchange of information to start the dialogs and keep them active, and to restart sessions that were interrupted or that were inactive for a prolonged period.

4. TCP / IP application layer protocols

   • The TCP / IP application protocols specify the format and control information necessary for many common Internet communication functions. Click on each application protocol in the figure to get more information about them.
   • The application layer protocols are used by both the source and target devices during a communication session. For communications to be carried out correctly, the application layer protocols that were implemented on the source and destination hosts must be compatible.

5. Client-server model

   • In the client-server model, the device that requests information is called "client", and the device that responds to the request is called "server". The client and server processes are considered part of the application layer. The client begins the exchange by requesting the data from the server, who responds by sending one or more data flows to the client. The application layer protocols describe the format of requests and responses between clients and servers. In addition to the actual transfer of data, this exchange may also require user authentication and the identification of a data file to be transferred.

6. Networks between pairs

   • In the peer-to-peer network model (P2P), data from an even device is accessed without using a dedicated server.
   • The P2P network model consists of two parts: P2P networks and P2P applications. Both parties have similar characteristics, but in practice they are very different.

7. Hypertext transfer protocol and hypertext markup language

   • When a web address or a uniform resource locator (URL) is written to a web browser, the browser establishes a connection to the web service running on the server using the HTTP protocol. The names that most people associate with web addresses are URLs and uniform resource identifiers (URI).
   • HTTP and HTTPS
     • HTTP is a request / response protocol. When a client, usually a web browser, sends a request to a web server, HTTP specifies the types of messages that are used for that communication. The three types of common messages are GET, POST and PUT

8. Email protocols

   • One of the main services that an ISP offers is email hosting. To execute the email on a PC or on another terminal, several services and applications are required, as shown in the figure. Email is a method to store and send that is used to send, store and retrieve electronic messages through a network. Email messages are stored in databases on mail servers.

9. Domain name service

   • In data networks, devices are labeled with numeric IP addresses to send and receive data through networks. Domain names were created to convert numeric addresses into a simple and recognizable name.