(print servers), or network traffic (network servers). A server receives a request and, after any necessary processing, the requested file is returned to the client. Typically, multiple client programs share the services of a common server. Figure shows client-server examples.
Content 7.3 Troubleshooting TCP/IP Application Layer Protocols 7.3.3 Terminals and consoles Telnet is the standard terminal emulation protocol in the TCP/IP protocol stack. Telnet is defined in RFC 854 and operates over the TCP port 23. Telnet and FTP were the first two services available on ARPANET. To understand how Telnet works, it is necessary to first make a distinction between console and terminal. A console refers to a keyboard and monitor that are directly connected to the computer system. In mainframe computing, a console was also referred to as a dumb terminal since it only operated by using the resources of the remote server. A microcomputer is now more commonly used as a console. All consoles require a terminal connection to enable users to log in to remote systems and use resources (for example, CPU, applications, and storage) as if they were connected to a local system. A terminal is a console that artificially emulates the physical hookup of a console. The destination host assumes it has a direct connection to the client since the terminal just provides a communication channel for the user's input and output. A terminal program is commonly used to connect to a central server over the network. The term "terminal emulator" refers to a terminal application that is implemented in software. Clients can use the Telnet program to establish a terminal connection. Other software such as HyperTerminal or TeraTerm can also be used and they typically offer more advanced features. The following lists commands that could be used to troubleshoot Telnet problems. Note: Debug output is normally sent to the console port. This means that if the connection is established through the Telnet port, the debug output will not be seen. Use the terminal monitor IOS command to redirect the output to any of the VTY ports. Keep the amount of debugging that is enabled to a minimum. Note: RDP Protocol (Terminal Server) is a little known Telnet port (port 3389) used by Remote Desktop Protocol (RDP). RDP is the remote Windows terminal protocol used by Microsoft Windows NT 4.0, Terminal Server Edition operating system and Windows 2000 Terminal Services. Keep this in mind when troubleshooting. Telnet for Troubleshooting
Network administrators often overlook Telnet as a troubleshooting tool. However, Telnetting to a host allows better verification of network status than just using ping. Telnet runs on top of the TCP protocol, so it establishes a more reliable indication of accessibility than ICMP echo requests can. It also tests higher-level functions of the destination host system. A server may be inaccessible for application layer functions, but still answers pings since those are handled by the lower layer protocols. Telnet also has an additional feature that makes it valuable for troubleshooting application layer protocols. Telnet client applications allow the user to select the destination port number to be used. It can be used to connect to other TCP ports on destination hosts to test out other functions. That means that Telnet can contact network application programs other than a Telnet server. This can be useful as a substitute for a client application program. For example, Telnetting to port 25 (SMTP) will verify that the e-mail server is answering. Telnet to port 80 (HTTP) to verify the Web server is answering. Source Telnet Interface
Finally, a useful IOS command to use when testing an access list is the ip telnet source-interface command. This specifies the IP address of an interface as the source address for Telnet connections. To reset the source address to the default for each connection, use the no form of this command. By default, Telnet will use the IP address of the closest interface to the destination as the source address. However, sometimes another interface may be preferred as the source. Conceptually, this is similar to specifying another source IP address when using an extended ping command. The following example forces the IP address for FastEthernet interface 0/1 as the source address for Telnet connections: Router(config)#ip telnet source-interface FastEthernet 0/1 Figure shows some IOS commands used to isolate Telnet problems.
Content 7.3 Troubleshooting TCP/IP Application Layer Protocols 7.3.4 Web traffic Hypertext Transfer Protocol (HTTP) is the protocol used to transfer the files that make up web pages. Although the HTTP specification allows for data to be transferred on port 80 using either TCP or UDP, most implementations use TCP. HTTPS is a secure version of the HTTP protocol. Aside from the initial connection and setup, HTTPS and HTTP are basically the same. The difference lies in the initial setup between client and server. HTTPS uses the Secure Socket Layer (SSL) protocol. SSL was created in order to secure credit card purchases over the Internet. It requires that both sides of a connection be authenticated and that data be encrypted and decrypted. It uses port 443 to initiate a secure connection. HTTP connectivity can be tested using any Telnet application that allows a port number to be specified by Telnetting to the IP address of the destination server using port 80. Note: By default characters are not echoed in MS Telnet upon successful connection to a Web Server unless 'local echo' is enabled in the preferences. The following table presents commands that can be used to troubleshoot a World Wide Web network application. A troubleshooter uses the information from these commands to isolate problems at the application layer that are related to the Web and the HTTP protocol.
Content 7.3 Troubleshooting TCP/IP Application Layer Protocols 7.3.5 Electronic mail Simple Mail Transport Protocol (SMTP) is used to transport e-mail messages in ASCII format using TCP between clients and servers. Other protocols such as Post Office Protocol (POP) or Internet Access Message Protocol (IMAP) are used to retrieve e-mails from mail servers. For example, assume User-A wants to send e-mail to User-B. When User-A clicks on the Send button, e-mail is sent to the local e-mail server using the SMTP protocol. The e-mail server will then send the e-mail using SMTP to User-B’s e-mail server. It remains stored there until User-B collects it. Later, User-B connects to the local e-mail server and downloads the e-mails using either POP3 or IMAP 4. POP v3 is the current version of the protocol and it is incompatible with earlier versions. POP3 downloads user e-mails to the local computer. For this reason, POP3 is best suited in situations where users retrieve their e-mail from the same computer. If users use different computers, their e-mails will likely be spread around several computers. IMAP v4 is another alternative that lets users download their e-mail at any time to any computer. Because these different protocols are used to send and receive mail, it is possible that mail clients can perform one task and not the other. Therefore, when verifying the configuration of a mail client, both the mail relay (SMTP) server and mail (POP or IMAP) servers should be verified. SMTP, IMAP, and POP connectivity can be tested using any Telnet application that allows a port number to be specified. Telnet to the IP address of the destination server using ports 25, 143, and 110