Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Slide 53Slide 54Slide 5526.1Chapter 26Remote Logging, Electronic Mail,and File TransferCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.26.226-1 REMOTE LOGGING26-1 REMOTE LOGGINGIt would be impossible to write a specific client/server It would be impossible to write a specific client/server program for each demand. The better solution is a program for each demand. The better solution is a general-purpose client/server program that lets a user general-purpose client/server program that lets a user access any application program on a remote computer.access any application program on a remote computer.TELNETTopics discussed in this section:Topics discussed in this section:26.3TELNET is a general-purpose client/server application program.Note26.4Figure 26.1 Local and remote log-in26.5Figure 26.2 Concept of NVT26.6Table 26.1 Some NVT control characters26.7Figure 26.3 An example of embedding26.8Table 26.2 Options26.9Table 26.3 NVT character set for option negotiation26.10Figure 26.4 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 26.126.11Figure 26.4 Example 26.1: Echo option26.12Table 26.4 Character set for suboptions26.13Figure 26.5 shows an example of suboption negotiation. In this example, the client wants to negotiate the type of the terminal.Example 26.226.14Figure 26.5 Example of suboption negotiation26.1526-2 ELECTRONIC MAIL26-2 ELECTRONIC MAILOne of the most popular Internet services is electronic One of the most popular Internet services is electronic mail (e-mail). The designers of the Internet probably mail (e-mail). The designers of the Internet probably never imagined the popularity of this application never imagined the popularity of this application program. Its architecture consists of several program. Its architecture consists of several components that we discuss in this chapter.components that we discuss in this chapter.ArchitectureUser AgentMessage Transfer Agent: SMTPMessage Access Agent: POP and IMAPWeb-Based MailTopics discussed in this section:Topics discussed in this section:26.16Figure 26.6 First scenario in electronic mail26.17When the sender and the receiver of an e-mail are on the same system,we need only two user agents.Note26.18Figure 26.7 Second scenario in electronic mail26.19When the sender and the receiver of an e-mail are on different systems, weneed two UAs and a pair of MTAs (client and server).Note26.20Figure 26.8 Third scenario in electronic mail26.21When the sender is connected to the mail server via a LAN or a WAN,we need two UAs and two pairsof MTAs (client and server).Note26.22Figure 26.9 Fourth scenario in electronic mail26.23Figure 26.10 Push versus pull in electronic email26.24When both sender and receiver are connected to the mail server via a LAN or a WAN, we need two UAs, two pairs of MTAs and a pair of MAAs.This is the most common situation today.Note26.25Figure 26.11 Services of user agent26.26Some examples of command-driven user agents are mail, pine, and elm.Note26.27Some examples of GUI-based user agents are Eudora, Outlook, and Netscape.Note26.28Figure 26.12 Format of an e-mail26.29Figure 26.13 E-mail address26.30Figure 26.14 MIME26.31Figure 26.15 MIME header26.32Table 26.5 Data types and subtypes in MIME26.33Table 26.6 Content-transfer-encoding26.34Figure 26.16 SMTP range26.35Figure 26.17 Commands and responses26.36Figure 26.18 Command format26.37Table 26.7 Commands26.38Table 26.8 Responses26.39Table 26.8 Responses (continued)26.40Let us see how we can directly use SMTP to send an e-mail and simulate the commands and responses we described in this section. We use TELNET to log into port 25 (the well-known port for SMTP). We then use the commands directly to send an e-mail. In this example, [email protected] is sending an e-mail to himself. The first few lines show TELNET trying to connect to the Adelphia mail server. After connection, we can type the SMTP commands and then receive the responses, as shown on the next slide. Note that we have added, for clarification, some comment lines, designated by the “=” signs. These lines are not part of the e-mail procedure.Example 26.326.41Example 26.3 (continued)$ telnet mail.adelphia.net 25Trying 68.168.78.100 . . .Connected to mail.adelphia.net (68.168.78.100).26.42Example 26.3 (continued)26.43Example 26.3 (continued)26.44Figure 26.19 POP3 and IMAP426.45Figure 26.20 The exchange of commands and responses in POP326.4626-3 FILE TRANSFER26-3 FILE TRANSFERTransferring files from one computer to another is one Transferring files from one computer to another is one of the most common tasks expected from a networking of the most common tasks expected from a networking or internetworking environment. As a matter of fact, or internetworking environment. As a matter of fact, the greatest volume of data exchange in the Internet the greatest volume of data exchange in the Internet today is due to file transfer. today is due to file transfer. File Transfer Protocol (FTP)Anonymous FTPTopics discussed in this section:Topics discussed in this section:26.47FTP uses the services of TCP. It needs two TCP connections.The well-known port 21 is used for the control connection and the well-known port 20 for the data connection.Note26.48Figure 26.21 FTP26.49Figure 26.22 Using the control connection26.50Figure 26.23 Using the data