Unformatted text preview: gment 10, with the urgent notification, contains the next byte of data from the server to
the client (sequence number 26727). It does not contain the "flush output" command byte.
The server can send this single byte in segment 10 because we saw in Section 22.2 that a
sender with data can always probe a closed window by sending 1 byte of data. The client's
TCP responds immediately in segment II with a zero window, but the receipt of the urgent
notification in segment 10 causes the client's TCP to notify the Rlogin client that the other
end of the connection has entered urgent mode.
Once the Rlogin client receives the urgent notification from its TCP, and starts reading the
data that's already waiting for it, the window opens up (segment 13). The data buffered by
the server is then sent (segments 14, 15, 17, and 18). The last of these contains the final
byte of urgent data (sequence number 30146), which contains the command byte from the
server to the client. When the client reads this byte it discards all the data that it read in file:///D|/Documents%20and%20Settings/bigini/Docum...ti/homenet2run/tcpip/tcp-ip-illustrated/telnet.htm (12 of 26) [12/09/2001 14.47.45] Chapter 26. Telnet and Rlogin: Remote Login segments 14, 15, 17, and 18, and flushes its terminal output queue. The next 2 bytes, in
segment 19, are the echo of the interrupt key: "^?". The final segment we show (21)
contains the shell prompt from the client.
This example shows how data can be buffered at both ends of the connection when the
client types the interrupt key. If this action only discarded the 3419 bytes buffered at the
server, without discarding the 4096 bytes at the client, these 4096 bytes of data, along with
whatever was buffered in the terminal output queue on the client, would be output. 26.4 Telnet Protocol
Telnet was designed to work between any host (i.e., any operating system) and any
terminal. Its specification in RFC 854 [Postel and Reynolds 1983a] defines the lowest
common denominator terminal, called the network virtual terminal (NVT). The NVT is an
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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.
- Spring '12