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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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The Interpret As Comm<strong>and</strong> (IAC) character is followed by a comm<strong>and</strong> code. If<br />

this comm<strong>and</strong> deals with option negotiation, the comm<strong>and</strong> will have a third byte<br />

to show the code for the referenced option. See Figure 13-4 for more details.<br />

Interpret<br />

as<br />

Comm<strong>and</strong><br />

Sample:<br />

Figure 13-4 Telnet: Internal Telnet comm<strong>and</strong> proposes negotiation about terminal type<br />

Table 13-3 shows some of the possible comm<strong>and</strong> codes.<br />

Table 13-3 Comm<strong>and</strong> codes<br />

Comm<strong>and</strong> Code Comments<br />

SE 240 End of sub-negotiation parameters.<br />

NOP 241 No operation.<br />

Data Mark 242 The data stream portion of a sync. This must<br />

always be accompanied by a <strong>TCP</strong> urgent<br />

notification.<br />

Break 243 NVT character BRK.<br />

Go Ahead 249 The GA signal.<br />

SB 250 Start of sub-negotiation of option indicated by the<br />

immediately following code.<br />

490 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong><br />

Comm<strong>and</strong><br />

Code<br />

Option<br />

Negotiated<br />

byte 1 byte 2<br />

byte 3<br />

255 253 24<br />

IAC<br />

WILL<br />

Terminal Type

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