TCP IP Illustrated

Option negotiation although telnet starts with both

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: data mark BRK 243 break IP 244 interrupt process AO 245 abort output AYT 246 are you there? EC 247 escape character EL 248 erase line GA 249 go ahead SB 250 suboption begin WILL 251 option negotiation (Figure 26.9) WONT 252 option negotiation IX) 253 option negotiation DONT 254 option negotiation IAC 255 data byte 255 Figure 26.8 Telnet commands, when preceded by IAC (255). Since many of these commands are rarely used, we describe the important commands when we encounter them in the discussion below and in the examples in the next section. Option Negotiation Although Telnet starts with both sides assuming an NVT, the first exchange that normally takes place across a Telnet connection is option negotiation. The option negotiation is symmetric - either side can send a request to the other. Either side can send one of four different requests for any given option. file:///D|/Documents%20and%20Settings/bigini/Docum...ti/homenet2run/tcpip/tcp-ip-illustrated/telnet.htm (14 of 26) [12/09/2001 14.47.45] Chapter 26. Telnet and Rlogin: Remote Login 1. 2. 3. 4. WILL. The sender wants to enable the option itself. DO. The sender wants the receiver to enable the option. WONT. The sender wants to disable the option itself. DONT. The sender wants the receiver to disable the option. Since the rules of Telnet allow a side to either accept or reject a request to enable an option (cases 1 and 2 above), but require a side to always honor a request to disable an option (cases 3 and 4 above), these four cases lead to the six scenarios shown in Figure 26.9. Sender 1. WILL 2. WILL 3. DO 4. DO 5. WONT 6. DONT Receiver -> <-> <-> <-> <-> <-> <- DO DONT WILL WONT DONT WONT Description sender wants to enable option receiver says OK sender wants to enable option receiver says NO sender wants receiver to enable option receiver says OK sender wants receiver to enable option receiver says NO sender wants to disable option receiver must say OK sender wants receiver to disable option receiver must say OK Figure 26.9 Six scenarios for Telnet option negotiation. Option negotiation requires 3 bytes: the IAC byte, follo...
View Full Document

Ask a homework question - tutors are online