TCP IP Illustrated

More than 30 different ftp commands can be sent by

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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 LIST filelist PASS password PORT n1,n2,n3,n4,n5,n6 QUIT RETR filename STOP filename SYST TYPE type 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. FTP Replies 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.

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