Separation of Control and Data

• FTP servers use well-known TCP port 21 for control

• • Log ons
  • Issuing commands and seeing results from the simple commands

• When a file is downloaded, the client and server determine a different set of port numbers to use for the file data

• • The control session on server port 21 is maintained
  • Two modes of operation:
  • • Active: The server uses port 20 and initiates a TCP session with an IP address and port provided by the client
    • Passive: The server allocates an ephemeral port, provides an IP and this port to this client, and the client initiates the session

• This concept also exists in most voice over IP (VoIP) implementations but is rare in strict data applications