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Unformatted text preview: requires a CR, LF pair at the end of each line (i.e., each
command or each reply).
The only Telnet commands (those that begin with IAC) that can be sent by the client to the
server are interrupt process (<IAC, IP>) and the Telnet synch signal (<IAC, DM> in
urgent mode). We'll see that these two Telnet commands are used to abort a file transfer
that is in progress, or to query the server while a transfer is in progress. Additionally, if the
server receives a Telnet option command from the client (WILL, WONT, DO, or DONT)
it responds with either DONT or WONT.
The commands are 3 or 4 bytes of uppercase ASCII characters, some with optional
arguments. More than 30 different FTP commands can be sent by the client to the server.
Figure 27.2 shows some of the commonly used commands, most of which we'll encounter
in this chapter.
Command Description ABOR
USER username abort previous FTP command and any data transfer
list files or directories
password on server
client IP address (nl.n2.n3.n4) and port (n5 x 256 + n6)
logoff from server
retrieve (get) a file
store (put) a file
server returns system type
specify file type: A for ASCII, I for image
usemame on server Figure 27.2 Common FTP commands.
We'll see in the examples in the next section that sometimes there is a one-to-one
correspondence between what the interactive user types and the FTP command sent across
the control connection, but for some operations a single user command results in multiple
FTP commands across the control connection.
The replies are 3-digit numbers in ASCII, with an optional message following the number.
The intent is that the software needs to look only at the number to determine how to
process the reply, and the optional string is for human consumption. Since the clients
normally output both the numeric reply and the message string, an interactive user can
determine what the reply says by just reading the string (and not have to memorize what
all the numeric reply codes mean). file:///D|/Documen...
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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