FTP File Transfer Protocol

Reference: RFC 959

FTP Objectives (from RFC 959)

• promote sharing of files
• encourage indirect use of remote computers
• shield user from variations in file storage
• transfer data reliably and efficiently
• “FTP, although usable directly by a user at a terminal, is designed mainly for use by programs”

The FTP Model

Control and Data Connections

• Control functions (commands) and reply codes are transferred over the control connection.
• All data transfer takes place over the data connection.
• The control connection must be “up” while data transfer takes place.

Control Connection

• The control connection is the “well known” service.
• The control connection uses the TELNET protocol.
• Commands and replies are all line oriented text (default is ASCII).

Standard Connection Model

Alternative Connection Model(Proxy FTP)

Access ControlCommands

• USER specify user
• PASS specify password
• CWD change directory
• CDUP change directory to parent
• QUIT logout

Transfer Parameter Commands

• PORT publish local data port
• PASV server should listen
• TYPE establish data representation
• MODE establish transfer mode
• STRU establish file structure
  

Service Commands

• RETR retrieve file
• STOR send file
• STOU send file and save as unique
• APPE send file and append
• ABOR abort previous service command
• PWD print working directory
• LIST transfer list of files over data link

FTP Replies

• All replies are sent over control connection.
• Replies are a single line containing
• 3 digit status code (sent as 3 numeric chars).
• text message.
• The FTP spec. includes support for multiline text replies.
• status code is used by programs, text is for humans.

FTP Reply Status Code

First digit of status code indicates type of reply:

• 1’: Positive Preliminary Reply (got it, but wait).‘
• 2’: Positive Completion Reply (success).‘
• 3’: Positive Intermediate Reply (waiting for more information).‘
• 4’: Transient Negative Completion (error - try again).‘
• 5’: Permanent Negative Reply (error - can’t do).

2nd digit indicates function groupings.

• 0’: Syntax (problem with command syntax).
• 1’: Information (reply to help or status cmds).‘
• 2’: Connections (problem with a connection).‘
• 3’: Authentication (problem with login).‘
• 4’: Unspecified.‘
• 5’: File system (related to file system).

3rd digit indicates specific problem within function group.

Data Transfer Modes

• STREAM: file is transmitted as a stream of bytes.
• BLOCK: file is transmitted as a series of blocks preceded by hearers containing count and descriptor code (EOF, EOR, restart marker).
• COMPRESSED: uses a simple compression scheme - compressed blocks are transmitted.

RFC 959 - Please Read !

• The RFC includes lots more information and many details including:
• parameters for commands
• lists of reply status codes
• protocol state diagrams
• support for a variety of file structures
• sample sessions