Data Communication Computer Network 简明教程

Application Layer Introduction

Application Layer

应用程序层是 OSI 和 TCP/IP 分层模型中最顶层。由于应用程序层与用户和用户应用程序交互非常重要，因此在两个分层模型中都存在此层。此层适用于参与通信系统的应用程序。

Application layer is the top most layer in OSI and TCP/IP layered model. This layer exists in both layered Models because of its significance, of interacting with user and user applications. This layer is for applications which are involved in communication system.

用户可能直接或不直接与应用程序交互。应用程序层是实际通信发起和反射的位置。因为此层位于层堆栈的顶部，所以它不向任何其他层提供服务。应用程序层借助传输层和所有其下层来向远程主机通信或传输其数据。

A user may or may not directly interacts with the applications. Application layer is where the actual communication is initiated and reflects. Because this layer is on the top of the layer stack, it does not serve any other layers. Application layer takes the help of Transport and all layers below it to communicate or transfer its data to the remote host.

当应用程序层协议想要与其在远程主机上的对等应用程序层协议通信时，它会将数据或信息传递给传输层。传输层借助下层的所有层完成其余工作。

When an application layer protocol wants to communicate with its peer application layer protocol on remote host, it hands over the data or information to the Transport layer. The transport layer does the rest with the help of all the layers below it.

application layer

在理解应用程序层及其协议时存在歧义。并非每个用户应用程序都可以放入应用程序层。除了与通信系统交互的应用程序外。例如，设计软件或文本编辑器不能被视为应用程序层程序。

There’is an ambiguity in understanding Application Layer and its protocol. Not every user application can be put into Application Layer. except those applications which interact with the communication system. For example, designing software or text-editor cannot be considered as application layer programs.

另一方面，当我们使用 Web 浏览器时，它实际上使用超文本传输协议 (HTTP) 与网络进行交互。HTTP 是应用程序层协议。

On the other hand, when we use a Web Browser, which is actually using Hyper Text Transfer Protocol (HTTP) to interact with the network. HTTP is Application Layer protocol.

另一个示例是文件传输协议，它帮助用户在网络中传输基于文本或二进制的文件。用户可以在基于 GUI 的软件（如 FileZilla 或 CuteFTP）中使用此协议，并且同一用户可以在命令行模式中使用 FTP。

Another example is File Transfer Protocol, which helps a user to transfer text based or binary files across the network. A user can use this protocol in either GUI based software like FileZilla or CuteFTP and the same user can use FTP in Command Line mode.

因此，无论您使用哪种软件，该软件使用的应用程序层中的协议都被考虑在内。DNS 是一个协议，它帮助用户应用程序协议（如 HTTP）完成其工作。

Hence, irrespective of which software you use, it is the protocol which is considered at Application Layer used by that software. DNS is a protocol which helps user application protocols such as HTTP to accomplish its work.

Functions of Application Layer

应用程序层的功能如下所述 −

The functions of application layer are explained below −

  1. *User Interface: * It represents the user interface to low-level layers and multiple application processes.

  2. *Security: * It is responsible for the execution of the security tests at the user entity points.

  3. *File Transfer: *The application layer supports the file transfer access and management (FTAM). It enables customers to create files in a remote system to retrieve the documents from a remote system and to handle or control the files in a remote system.

  4. *E-mail: *It supports a basis for email forwarding and string.

  5. *Database Access: *It supports distributed database sources and global data about several objects and functions.

  6. Addressing: It is used for the connection between user and server. There is a requirement for addressing. When a user requests the server, the request includes the server address and its address. The server responds to the user request and the request consists of the destination address, i.e., client address.

  7. Directory Services: An application includes a distributed database that supports the global data about multiple objects and functions.