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

2.  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) 2

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4.  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) 4

5.  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 1 catfileaIACEC1 Internet Computing (CS-413) 5

6.  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) 6

7.  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) 7

8.  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) 8

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

10.  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) 10

11.  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) 11