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4 Network Applications
Now that students have seen what happens to data packets as they travel the presentation layer, it is time to look at the last layer in which data packets travel through before reaching their final destination. The last layer or Layer 7 of the OSI model is referred to as the application layer. The application layer is the closest to the end-user, who interfaces directly with Layer 7 every time an e-mail is sent or received. You will see how the application layer deals with data packets from client-server applications, domain name services, and network applications by examining the following:
• Client-Server
• Redirectors
• Domain Name System
• Telnet
• FTP
• HTTP
4.1 Basics of the Application Layer
4.1.1 Application processes
In the context of the OSI reference model, the application layer (Layer 7) supports the communicating component of an application. The application layer is responsible for the following:
• Identifying and establishing the availability of intended communication partners
• Synchronizing cooperating applications
• Establishing agreement on procedures for error recovery
• Controlling data integrity
The application layer is the OSI layer closest to the end system. This determines whether sufficient resources exist for communication between systems. Without the application layer, there would be no network communication support.
The application layer does not provide services to any other OSI layer. It does provide services to application processes lying outside the scope of the OSI model. Examples of such application processes include spreadsheet programs, word processing programs, and banking terminal programs. Additionally, the application layer provides a direct interface for the rest of the OSI model by using network applications (for example, Browser, e-mail, FTP, Telnet), or an indirect interface by using standalone applications (for example, word processors, spreadsheets, presentation managers) with a network redirector.
参考答案:4 种网络应用 即然学生看见了什么发生在数据包当他们旅行表示层, 是时间看数据包旅行通过在到达他们的最终目的地之前的最后层数。最后层数或层数7 开放系统互连模式指应用层。应用层是最接近终端用户, 协调直接地与层数7 电子邮件被送或每次被接受。您将看见怎么应用层处理数据包从客户服务器应用、域名服务, 和网络应用由审查以下: ．客户服务器 ．Redirectors ．域名系统 ．电子邮件 ．Telnet ．FTP ．HTTP 4.1 应用层的基本 4.1.1 应用过程 就OSI 模型的状况, 应用层(层数7) 支持应用的通信的组分。应用层负责对以下: ．辨认和建立意欲的通信的可及性成为伙伴 ．同步合作的应用 ．建立关于规程的协议为错误校正 ．控制数据正直 应用层是开放系统互连层最接近结束系统。这确定是否充足的资源为通信存在在系统之间。没有应用层, 有会是没有网络通信支持。 应用层不提供服务对其他开放系统互连层。它提供服务对应用过程说谎在开放系统互连模式的范围之外。例子的这样应用过程包括空白表格程序、文字处理程序, 和银行业务终端程序。另外, 应用层为开放系统互连模式的剩余供给一个直接接口由使用网络应用(例如, 浏览器, 电子邮件, FTP, Telnet), 或一个间接接口由使用独立应用(例如, 文字处理软件、报表, 介绍经理) 网络redirector 。 呵呵