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Unformatted text preview: e command bytes are sent in the normal stream of data. The bytes used to
denote these in-band commands, 0xff, are chosen because we are unlikely to type keys
that generate these bytes. But the Rlogin method is not perfect. If we could generate two
consecutive bytes of 0xff from our keyboard, followed by two ASCII s's, the next 8 bytes
we type will be interpreted as window sizes.
The Rlogin commands from the server to the client, which we described in Figure 26.4, are
termed out-of-band signaling since the technique used is called "out-of-band data" by most
APIs. But recall our discussion of TCP's urgent mode in Section 20.8 where we said that
urgent mode is not out-of-band data, and the command byte is sent in the normal stream of
data, pointed to by the urgent pointer.
Since in-band signaling is used from the client to the server, the server must examine every
byte that it receives from the client, looking for two consecutive bytes of 0xff. But with
out-of-band signaling used from the server to the client, the client does not need to examine
the data that it receives from the server, until the server enters urgent mode. Even in urgent
mode, the client only needs to look at the byte pointed to by the urgent pointer. Since the
ratio of bytes from the client to server, versus from the server to client, is about 1:20, it
makes sense to use in-band signaling for the low-volume data flow (client to server) and
out-of-band signaling for the higher volume data flow (server to client).
Normally everything we type to the Rlogin client is sent to the server. Occasionally,
however, we want to talk directly to the Rlogin client program itself, and not have what we
type sent to the server. This is done by typing a tilde (~) as the first character of a line,
followed by one of the following four characters:
4. A period terminates the client.
The end-of-file character (often Control-D) terminates the client.
The job control suspend character (often Control-Z) suspends t...
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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