tailieunhanh - Lecture Data communications and networking: Chapter 26 - Behrouz A. Forouzan

Chapter 26 - Remote logging, electronic mail, and file transfer. Chapter 26 discusses three common applications in the Internet: remote login, electronic mail, and file transfer. A remote login application allows the user to remotely access the resources of a system. An electronic mail application simulates the tasks ofsnail mail at a much higher speed. A file transfer application transfers files between remote systems. | Chapter 26 Remote Logging, Electronic Mail, and File Transfer Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 26. 26-1 REMOTE LOGGING It would be impossible to write a specific client/server program for each demand. The better solution is a general-purpose client/server program that lets a user access any application program on a remote computer. TELNET Topics discussed in this section: 26. TELNET is a general-purpose client/server application program. Note 26. Figure Local and remote log-in 26. Figure Concept of NVT 26. Table Some NVT control characters 26. Figure An example of embedding 26. Table Options 26. Table NVT character set for option negotiation 26. Figure shows an example of option negotiation. In this example, the client wants the server to echo each character sent to the server. The echo option is enabled by the server because it is the server that sends the characters back to the user terminal. Therefore, the client should request from the server the enabling of the option using DO. The request consists of three characters: IAC, DO, and ECHO. The server accepts the request and enables the option. It informs the client by sending the three-character approval: IAC, WILL, and ECHO. Example 26. Figure Example : Echo option 26. Table Character set for suboptions 26. Figure shows an example of suboption negotiation. In this example, the client wants to negotiate the type of the terminal. Example 26. Figure Example of suboption negotiation 26. 26-2 ELECTRONIC MAIL One of the most popular Internet services is electronic mail (e-mail). The designers of the Internet probably never imagined the popularity of this application program. Its architecture consists of several components that we discuss in this chapter. Architecture User Agent Message Transfer Agent: SMTP Message Access Agent: POP . | Chapter 26 Remote Logging, Electronic Mail, and File Transfer Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 26. 26-1 REMOTE LOGGING It would be impossible to write a specific client/server program for each demand. The better solution is a general-purpose client/server program that lets a user access any application program on a remote computer. TELNET Topics discussed in this section: 26. TELNET is a general-purpose client/server application program. Note 26. Figure Local and remote log-in 26. Figure Concept of NVT 26. Table Some NVT control characters 26. Figure An example of embedding 26. Table Options 26. Table NVT character set for option negotiation 26. Figure shows an example of option negotiation. In this example, the client wants the server to echo each character sent to the server. The echo option is enabled by the server because it is the server that sends .