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Server to Client Commands
We can now summarize the four commands that the Rlogin server can send to the client
across the TCP connection. The problem is that only a single TCP connection is used, so
the server needs to mark these command bytes so the client knows to interpret them as
commands, and not display the bytes on the terminal. TCP's urgent mode is used for this
When the server sends a command to the client, the server enters urgent mode with the last
byte of urgent data being the command byte from the server. When the client receives the
urgent mode notification, it reads from the connection, saving the data until the command
byte (the last byte of urgent data) is encountered. The data that's saved by the client can be file:///D|/Documents%20and%20Settings/bigini/Documenti/homenet2run/tcpip/tcp-ip-illustrated/telnet.htm (5 of 26) [12/09/2001 14.47.45] Chapter 26. Telnet and Rlogin: Remote Login displayed on the terminal, or discarded, depending on the command. Figure 26.4 describes
the four command bytes.
Byte Description Flush output. The client discards all the data received from the server, up through
the command byte (the last byte of urgent data). The client also discards any
pending terminal output that may be buffered. The server sends this command
when it receives the interrupt key from the client.
0x10 The client stops performing flow control.
0x20 The client resumes flow control processing.
The client responds immediately by sending the current window size to the server,
0x80 and notifies the server in the future if the window size changes. This command is
normally sent by the server immediately after the connection is established.
Figure 26.4 Rlogin commands from the server to the client.
One reason for sending these commands using TCP's urgent mode is that the first command
("flush output") needs to be sent to the client even if the flow of data from the server to the
client is stopped by TCP's windowed flow control. Th...
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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