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telnet

telnet. TCP/IP Suit by Behrouz Fourozan 4 th Ed. concept. General purpose client server program Developed when time sharing systems were being used Time sharing environment Large computer with multiple users Interaction with user through terminals Login Local login Remote login.

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telnet

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  1. telnet TCP/IP Suit by Behrouz Fourozan 4th Ed

  2. concept • General purpose client server program • Developed when time sharing systems were being used • Time sharing environment • Large computer with multiple users • Interaction with user through terminals • Login • Local login • Remote login Internet Computing (CS-413)

  3. Netork virtual terminal Internet Computing (CS-413)

  4. NVT characterset • Data characters • 8b long • MSB =0 • Control characters • 8b long • MSB=1 • Examples EOF, EOR, SE, NOP, DM, IP, AO, GA, will, do, dont, IAC Internet Computing (CS-413)

  5. embedding • Uses TCP port 23 for server • Client may use any port • Same connection for data and control • Control character is preceded by Interpret as Control (IAC) • Eg cat file1  mistyped to cat filea<backspace> 1 Internet Computing (CS-413)

  6. options • telnet lets client and server negotiate option before or during the use of the service • Options are extra features available to a user with more sophisticated terminal • Examples binary, ECHO, Suppress go ahead, status, timing marks, terminal type, terminal speed, line mode Internet Computing (CS-413)

  7. Option negotiation • To use options, option negotiation is required • Four control characters • WILL • WONT • DO • DONT • Enabling an option • Some can be enabled only by server, some only by client and some by both • Two possibilities • Offer or request Internet Computing (CS-413)

  8. Option negotiation • disabling • Two possibilities • Offer to disable • Request to disable • Symmetry • Client and servers are given equal opportunity • If one party wants an option enabled, it may offer or request Internet Computing (CS-413)

  9. Suboption negotiation • Some options require additional information • Example define type or speed of a terminal • SE – suboption end • SB – suboption begin Internet Computing (CS-413)

  10. Controlling the server • Client may be controlled by ctrl+C or back space etc • Server is controlled through special characters • IP – interrupt process • AO- Abort output • AYT – are you there • EC- erase last character • EL- erase line Internet Computing (CS-413)

  11. Out of band signaling • Control characters are preceeded by IAC and sent out of order • TCP segment is marked with URG bit and Urgent data pointer • Urgent data points to DM (data mark) character. • DM tells where the out of order data is present Internet Computing (CS-413)

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