TCP IP Illustrated

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Unformatted text preview: wed by the byte for WILL, DO, WONT, or DONT, followed by an ID byte specifying the option to enable or disable. Currently more than 40 different options can be negotiated. The Assigned Numbers RFC specifies the value for the option byte and the relevant RFC that describes the option. Figure 26.10 shows the option codes that we'll see in this chapter. Option ID (decimal) Name RFC 1 echo 857 3 suppress go ahead 858 5 status 859 6 timing mark 860 24 terminal type 1091 31 window size 1073 file:///D|/Documents%20and%20Settings/bigini/Docum...ti/homenet2run/tcpip/tcp-ip-illustrated/telnet.htm (15 of 26) [12/09/2001 14.47.45] Chapter 26. Telnet and Rlogin: Remote Login 32 terminal speed remote 1079 33 flow control 1372 34 linemode 1184 36 environment variables 1408 Figure 26.10 Telnet option codes discussed in the text. Telnet option negotiation, like most of the Telnet protocol, is intended to be symmetrical. Either end can initiate the negotiation of an option. But remote login is not a symmetrical application. The client performs some tasks, and the server performs others. We'll see as we look at some of the Telnet options that some are intended only for the client (asking to enable linemode, for example), and some are only for the server. Suboption Negotiation Some options require more information than just "enable" or "disable." Specifying the terminal type is an example: an ASCII string must be sent by the client identifying the type of terminal. To handle these options, suboption negotiation is defined. RFC 1091 [VanBokkelen 1989] defines the suboption negotiation for the terminal type. First one side (normally the client) asks to enable the option by sending the 3-byte sequence <IAC, WILL, 24> where 24 (decimal) is the option ID for the terminal type option. If the receiver (the server) says OK, its response is <IAC, DO, 24> The server then sends <IAC, SB, 24, 1, IAC, SE> asking the client for its terminal type. SB is the suboption-begin command. The next byte of 24 identifies that this i...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.

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