Некоторые выдержки из курса CCNA 1

The Application layer, Layer seven is the top layer of both the OSI and TCP/IP models. It is the layer that provides the interface between the applications we use to communicate and the underlying network over which our messages are transmitted. Application layer protocols are used to exchange data between programs running on the source and destination hosts. There are many Application layer protocols and new protocols are always being developed.

Although the TCP/IP protocol suite was developed prior to the definition of the OSI model, the functionality of the TCP/IP Application layer protocols fit roughly into the framework of the top three layers of the OSI model: Application, Presentation and Session layers.

Most TCP/IP Application layer protocols were developed before the emergence of personal computers, graphical user interfaces and multimedia objects. As a result, these protocols implement very little of the functionality that is specified in the OSI model Presentation and Session layers.

The Presentation Layer

- The Presentation layer has three primary functions:
- Coding and conversion of Application layer data to ensure that data from the source device can be interpreted by the appropriate application on the destination device.

Presentation layer implementations are not typically associated with a particular protocol stack. The standards for video and graphics are examples. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG). QuickTime is an Apple Computer specification for video and audio, and MPEG is a standard for video compression and coding.

Among the well-known graphic image formats are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF). GIF and JPEG are compression and coding standards for graphic images, and TIFF is a standard coding format for graphic images.

The Session Layer

As the name of the Session layer implies, functions at this layer create and maintain dialogs between source and destination applications. The Session layer handles the exchange of information to initiate dialogs, keep them active, and to restart sessions that are disrupted or idle for a long period of time.

Most applications, like web browsers or e-mail clients, incorporate functionality of the OSI layers 5, 6 and 7.

Application layer Services

Other programs may need the assistance of Application layer services to use network resources, like file transfer or network print spooling. Though transparent to the user, these services are the programs that interface with the network and prepare the data for transfer. Different types of data - whether it is text, graphics, or video - require different network services to ensure that it is properly prepared for processing by the functions occurring at the lower layers of OSI model.

Each application or network service uses protocols which define the standards and data formats to be used. Without protocols, the data network would not have a common way to format and direct data. In order to understand the function of various network services, it is necessary to become familiar with the underlying protocols that govern their operation.

In the OSI model, applications that interact directly with people are considered to be at the top of the stack, as are the people themselves. Like all layers within the OSI model, the Application layer relies on the functions of the lower layers in order to complete the communication process. Within the Application layer, protocols specify what messages are exchanged between the source and destination hosts, the syntax of the control commands, the type and format of the data being transmitted, and the appropriate methods for error notification and recovery.

Application layer protocols are used by both the source and destination devices during a communication session. In order for the communications to be successful, the Application layer protocols implemented on the source and destination host must match.

Protocols establish consistent rules for exchanging data between applications and services loaded on the participating devices. Protocols specify how data inside the messages is structured and the types of messages that are sent between source and destination. These messages can be requests for services, acknowledgments, data messages, status messages, or error messages. Protocols also define message dialogues, ensuring that a message being sent is met by the expected response and the correct services are invoked when data transfer occurs.

Many different types of applications communicate across data networks. Therefore, Application layer services must implement multiple protocols to provide the desired range of communication experiences. Each protocol has a specific purpose and contains the characteristics required to meet that purpose. The right protocol details in each layer must be followed so that the functions at one layer interface properly with the services in the lower layer.

Applications and services may also use multiple protocols in the course of a single conversation. One protocol may specify how to establish the network connection and another describe the process for the data transfer when the message is passed to the next lower layer.

Data transfer from a client to a server is referred to as an upload and data from a server to a client as a download.

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