FTP Internal Command Groups and Protocol Commands
Once a connection is established between an FTP server and user, all communication to manage the operation of the protocol takes place over the control channel. The user-PI sends protocol commands to the server-PI, which processes them and takes appropriate action. The server-PI responds with reply codes to tell the user-PI the result of the commands it issued and convey other important information.
Interestingly, the actual transmission of FTP commands over the control channel is done using specifications based on the Telnet Protocol. You may recall from the "FTP Overview, History, and Standards" section earlier in this chapter that Telnet and FTP are two of the very oldest TCP/IP applications, ...
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