TCP/IP Protocol Suite 1 Chapter 18 Upon completion you will be able to: Remote Login: Telnet Understand how TELNET works Understand the role of NVT in.

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Presentation transcript:

TCP/IP Protocol Suite 1 Chapter 18 Upon completion you will be able to: Remote Login: Telnet Understand how TELNET works Understand the role of NVT in a TELNET communication Understand TELNET option and suboption negotiation Know how control characters are used Know the TELNET modes of operation Objectives

TCP/IP Protocol Suite 2 TELNET is a general-purpose client-server application program. Note:

TCP/IP Protocol Suite CONCEPT TELNET enables the establishment of a connection to a remote system in such a way that the local terminal appears to be a terminal at the remote system. The topics discussed in this section include: Time-Sharing Environment Login

TCP/IP Protocol Suite 4 Figure 18.1 Local login

TCP/IP Protocol Suite 5 Figure 18.2 Remote login

TCP/IP Protocol Suite NETWORK VIRTUAL TERMINAL (NVT) Via a universal interface called the Network Virtual Terminal (NVT) character set, the TELNET client translates characters (data or commands) that come from the local terminal into NVT form and delivers them to the network. The TELNET server translates data and commands from NVT form into the form acceptable by the remote computer.

TCP/IP Protocol Suite 7 Figure 18.3 Concept of NVT

TCP/IP Protocol Suite NVT CHARACTER SET NVT uses two sets of characters, one for data and one for control. Both are 8-bit bytes. The topics discussed in this section include: Data Characters Control Characters

TCP/IP Protocol Suite 9 Figure 18.4 Format of data characters

TCP/IP Protocol Suite 10 Figure 18.5 Format of control characters

TCP/IP Protocol Suite 11 Table 18.1 Some NVT control characters

TCP/IP Protocol Suite EMBEDDING The same connection is used by TELNET for sending both data and control characters. TELNET accomplishes this by embedding the control characters in the data stream.

TCP/IP Protocol Suite 13 Figure 18.6 An example of embedding

TCP/IP Protocol Suite OPTIONS TELNET lets the client and server negotiate options before or during the use of the service. Options are extra features available to a user with a more sophisticated terminal.

TCP/IP Protocol Suite 15 Table 18.2 Options

TCP/IP Protocol Suite OPTION NEGOTIATION To use any of the options first requires option negotiation between the client and the server. Four control characters are used for this purpose. The topics discussed in this section include: Enabling an Option Disabling an Option Symmetry

TCP/IP Protocol Suite 17 Table 18.3 NVT character set for option negotiation

TCP/IP Protocol Suite 18 Figure 18.7 Offer to enable an option

TCP/IP Protocol Suite 19 Figure 18.8 Request to enable an option

TCP/IP Protocol Suite 20 Figure 18.9 Offer to disable an option

TCP/IP Protocol Suite 21 Figure Request to disable an option

TCP/IP Protocol Suite 22 Figure shows an example of option negotiation. In this example, the client wants the server to echo each character sent to the server. In other words, when a character is typed at the user keyboard terminal, it goes to the server and is sent back to the screen of the user before being processed. 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 1 See Next Slide

TCP/IP Protocol Suite 23 Figure Example 1: Echo option

TCP/IP Protocol Suite SUBOPTION NEGOTIATION Some options require additional information. The client and server agree through a process called suboption negotiation.

TCP/IP Protocol Suite 25 Table 18.4 NVT character set for suboption negotiation

TCP/IP Protocol Suite 26 Figure Example of suboption negotiation

TCP/IP Protocol Suite CONTROLLING THE SERVER Control characters can be used to control the remote server.

TCP/IP Protocol Suite 28 Table 18.5 Characters used to control the application program running on remote server

TCP/IP Protocol Suite 29 Figure Example of interrupting an application program

TCP/IP Protocol Suite OUT-OF-BAND SIGNALING To make control characters effective in special situations, TELNET uses out-of-band signaling, a technique in which the control characters are preceded by IAC and are sent out of order to the remote process.

TCP/IP Protocol Suite 31 Figure Out-of-band signaling

TCP/IP Protocol Suite ESCAPE CHARACTER When a user wants characters interpreted by the client instead of the server, he can use an escape character, normally Ctrl+] (shown as ^]).

TCP/IP Protocol Suite 33 Figure Two different interruptions

TCP/IP Protocol Suite MODE OF OPERATION Most TELNET implementations operate in one of three modes: default mode, character mode, or line mode. The topics discussed in this section include: Default Mode Character Mode Line Mode

TCP/IP Protocol Suite 35 In this example, we use the default mode to show the concept and its deficiencies even though it is almost obsolete today. The client and the server negotiate the terminal type and terminal speed and then the server checks the login and password of the user (see Figure 18.16). Example 2 See Next Slide

TCP/IP Protocol Suite 36 Figure Example 2

TCP/IP Protocol Suite 37 In this example, we show how the client switches to the character mode. This requires that the client request the server to enable the SUPPRESS GO AHEAD and ECHO options (see Figure 18.17). Example 3 See Next Slide

TCP/IP Protocol Suite 38 Figure Example 3

TCP/IP Protocol Suite USER INTERFACE The operating system (UNIX, for example) defines an interface to TELNET with user-friendly commands. The interface is responsible for translating the user-friendly commands to the TELNET commands.

TCP/IP Protocol Suite 40 Table 18.6 Examples of interface commands

TCP/IP Protocol Suite SECURITY ISSUE TELNET suffers from security problems. TELNET requires a login name and password (when exchanging text). A microcomputer connected to a broadcast LAN can easily eavesdrop using snooper software to capture a login name and the corresponding password (even if it is encrypted).