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File Transfer Protocol (FTP)
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Introduction
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  • Purpose:
    • Transfer files
  • Specification:
    • RFC 959
Channels/Connections in FTP
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  • Control Channel:
    • Uses port 21
    • For things like: user ID, passwords, commands
  • Data Channel:
    • Uses port 20
    • For the actual file
Modes
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  • Active Mode:
    • The local host initiates communication on the control channel
    • The remote host then initiates communication on the data channel
  • Passive Mode:
    • The local host initiates communication on the control channel
    • The remote host responds with host information to use to create the data channel
    • The local host then uses that information to initiate communication on the data channel
Commands
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  • Properties:
    • Sent from client to server
    • Use the control channel
    • Sent in ASCII
  • Examples:
    • USER SP username CRLF
    • PASS SP password CRLF
    • CWD SP directory CRLF
    • LIST [SP pathname] CRLF
    • RETR SP pathname CRLF
    • STOR SP pathname CRLF
Reply Codes
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  • First Digit:
    • 1 - Positive preliminary reply
    • 2 - Positive completion reply
    • 3 - Positive intermediate reply
    • 4 - Transient negative completion reply
    • 5 - Permanent negative completion reply
  • Second Digit:
    • 0 - Syntax error
    • 1 - Request for information
    • 2 - Related to the connection
    • 3 - Related to authentication/accounting
    • 5 - File system status
  • Third Digit:
    • Details
State Information
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  • The Concept:
    • A sequence of commands/responses (usually starting with USER and ending with QUIT) defines a session
    • The protocol allows for an infinite number of possible sequences of commands
  • The Implication:
    • The server must maintain information about the user (e.g., whether the password has been transmitted, the current working directory)
There's Always More to Learn
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