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Unformatted text preview: ode and again we see an implementation (SVR4) that uses the older, but more
common interpretation, where the urgent pointer specifies the sequence number plus one of
the last byte of urgent data. The client responds with 12 bytes of data: 2 bytes of 0xff, 2
bytes of s, and 4 16-bit values.
The next four segments from the server (10, 12, 14, and 16) are the operating system
greeting from the server. This is followed by the 7-byte shell prompt on the server: "svr4
% " in segment 18.
The data entered by the client is then sent 1 byte at a time, as shown in Figure 19.2. The
connection can be closed by either end. If we type a command that causes the shell running
on the server to terminate, the server's end does the active close. If we type an escape to the
Rlogin client (normally a tilde), followed by a period or our end-of-file character, the client
does the active close.
The client port number in Figure 26.5 is 1023, which is within the range controlled by the IANA (Section
1.9). The Rlogin protocol requires the client to have a port number less than 1024, termed a reserved port.
On Unix systems, a client cannot obtain a reserved port unless the process has superuser privilege. This is
part of the authentication between the client and server, which allows the user to login without entering a
password. [Stevens 1990] discusses these reserved ports and the authentication used between the client and
server in more detail. Client Interrupt Key
Let's look at another example, this one involving TCP's urgent mode, when the flow of data file:///D|/Documents%20and%20Settings/bigini/Documenti/homenet2run/tcpip/tcp-ip-illustrated/telnet.htm (9 of 26) [12/09/2001 14.47.45] Chapter 26. Telnet and Rlogin: Remote Login has been stopped and we type the interrupt key. This example brings together many of the
TCP algorithms we described earlier: urgent mode, silly window avoidance, windowed
flow control, and the persist timer. We start the client on the host sun. We login to bsdi,
output a big text fi...
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- Spring '12