Section 5. DECLARATIVE SPECIFICATIONS


   5.1.  MINIMUM IMPLEMENTATION

      In order to make FTP workable without needless error messages, the
      following minimum implementation is required for all servers:

         TYPE - ASCII Non-print
         MODE - Stream
         STRUCTURE - File, Record
         COMMANDS - USER, QUIT, PORT,
                    TYPE, MODE, STRU,
                      for the default values
                    RETR, STOR,
                    NOOP.

      The default values for transfer parameters are:

         TYPE - ASCII Non-print
         MODE - Stream
         STRU - File

      All hosts must accept the above as the standard defaults.









   5.2.  CONNECTIONS

      The server protocol interpreter shall "listen" on Port L.  The
      user or user protocol interpreter shall initiate the full-duplex
      control connection.  Server- and user- processes should follow the
      conventions of the Telnet protocol as specified in the
      ARPA-Internet Protocol Handbook [1].  Servers are under no
      obligation to provide for editing of command lines and may require
      that it be done in the user host.  The control connection shall be
      closed by the server at the user's request after all transfers and
      replies are completed.

      The user-DTP must "listen" on the specified data port; this may be
      the default user port (U) or a port specified in the PORT command.
      The server shall initiate the data connection from his own default
      data port (L-1) using the specified user data port.  The direction
      of the transfer and the port used will be determined by the FTP
      service command.

      Note that all FTP implementation must support data transfer using
      the default port, and that only the USER-PI may initiate the use
      of non-default ports.

      When data is to be transferred between two servers, A and B (refer
      to Figure 2), the user-PI, C, sets up control connections with
      both server-PI's.  One of the servers, say A, is then sent a PASV
      command telling him to "listen" on his data port rather than
      initiate a connection when he receives a transfer service command.
      When the user-PI receives an acknowledgment to the PASV command,
      which includes the identity of the host and port being listened
      on, the user-PI then sends A's port, a, to B in a PORT command; a
      reply is returned.  The user-PI may then send the corresponding
      service commands to A and B.  Server B initiates the connection
      and the transfer proceeds.  The command-reply sequence is listed
      below where the messages are vertically synchronous but
      horizontally asynchronous:














         User-PI - Server A                User-PI - Server B
         ------------------                ------------------
         
         C->A : Connect                    C->B : Connect
         C->A : PASV
         A->C : 227 Entering Passive Mode. A1,A2,A3,A4,a1,a2
                                           C->B : PORT A1,A2,A3,A4,a1,a2
                                           B->C : 200 Okay
         C->A : STOR                       C->B : RETR
                    B->A : Connect to HOST-A, PORT-a

                                Figure 3

      The data connection shall be closed by the server under the
      conditions described in the Section on Establishing Data
      Connections.  If the data connection is to be closed following a
      data transfer where closing the connection is not required to
      indicate the end-of-file, the server must do so immediately.
      Waiting until after a new transfer command is not permitted
      because the user-process will have already tested the data
      connection to see if it needs to do a "listen"; (remember that the
      user must "listen" on a closed data port BEFORE sending the
      transfer request).  To prevent a race condition here, the server
      sends a reply (226) after closing the data connection (or if the
      connection is left open, a "file transfer completed" reply (250)
      and the user-PI should wait for one of these replies before
      issuing a new transfer command).

      Any time either the user or server see that the connection is
      being closed by the other side, it should promptly read any
      remaining data queued on the connection and issue the close on its
      own side.

   5.3.  COMMANDS

      The commands are Telnet character strings transmitted over the
      control connections as described in the Section on FTP Commands.
      The command functions and semantics are described in the Section
      on Access Control Commands, Transfer Parameter Commands, FTP
      Service Commands, and Miscellaneous Commands.  The command syntax
      is specified here.

      The commands begin with a command code followed by an argument
      field.  The command codes are four or fewer alphabetic characters.
      Upper and lower case alphabetic characters are to be treated
      identically.  Thus, any of the following may represent the
      retrieve command:



                  RETR    Retr    retr    ReTr    rETr

      This also applies to any symbols representing parameter values,
      such as A or a for ASCII TYPE.  The command codes and the argument
      fields are separated by one or more spaces.

      The argument field consists of a variable length character string
      ending with the character sequence <CRLF> (Carriage Return, Line
      Feed) for NVT-ASCII representation; for other negotiated languages
      a different end of line character might be used.  It should be
      noted that the server is to take no action until the end of line
      code is received.

      The syntax is specified below in NVT-ASCII.  All characters in the
      argument field are ASCII characters including any ASCII
      represented decimal integers.  Square brackets denote an optional
      argument field.  If the option is not taken, the appropriate
      default is implied.
































      5.3.1.  FTP COMMANDS

         The following are the FTP commands:

            USER <SP> <username> <CRLF>
            PASS <SP> <password> <CRLF>
            ACCT <SP> <account-information> <CRLF>
            CWD  <SP> <pathname> <CRLF>
            CDUP <CRLF>
            SMNT <SP> <pathname> <CRLF>
            QUIT <CRLF>
            REIN <CRLF>
            PORT <SP> <host-port> <CRLF>
            PASV <CRLF>
            TYPE <SP> <type-code> <CRLF>
            STRU <SP> <structure-code> <CRLF>
            MODE <SP> <mode-code> <CRLF>
            RETR <SP> <pathname> <CRLF>
            STOR <SP> <pathname> <CRLF>
            STOU <CRLF>
            APPE <SP> <pathname> <CRLF>
            ALLO <SP> <decimal-integer>
                [<SP> R <SP> <decimal-integer>] <CRLF>
            REST <SP> <marker> <CRLF>
            RNFR <SP> <pathname> <CRLF>
            RNTO <SP> <pathname> <CRLF>
            ABOR <CRLF>
            DELE <SP> <pathname> <CRLF>
            RMD  <SP> <pathname> <CRLF>
            MKD  <SP> <pathname> <CRLF>
            PWD  <CRLF>
            LIST [<SP> <pathname>] <CRLF>
            NLST [<SP> <pathname>] <CRLF>
            SITE <SP> <string> <CRLF>
            SYST <CRLF>
            STAT [<SP> <pathname>] <CRLF>
            HELP [<SP> <string>] <CRLF>
            NOOP <CRLF>












      5.3.2.  FTP COMMAND ARGUMENTS

         The syntax of the above argument fields (using BNF notation
         where applicable) is:

            <username> ::= <string>
            <password> ::= <string>
            <account-information> ::= <string>
            <string> ::= <char> | <char><string>
            <char> ::= any of the 128 ASCII characters except <CR> and
            <LF>
            <marker> ::= <pr-string>
            <pr-string> ::= <pr-char> | <pr-char><pr-string>
            <pr-char> ::= printable characters, any
                          ASCII code 33 through 126
            <byte-size> ::= <number>
            <host-port> ::= <host-number>,<port-number>
            <host-number> ::= <number>,<number>,<number>,<number>
            <port-number> ::= <number>,<number>
            <number> ::= any decimal integer 1 through 255
            <form-code> ::= N | T | C
            <type-code> ::= A [<sp> <form-code>]
                          | E [<sp> <form-code>]
                          | I
                          | L <sp> <byte-size>
            <structure-code> ::= F | R | P
            <mode-code> ::= S | B | C
            <pathname> ::= <string>
            <decimal-integer> ::= any decimal integer





















   5.4.  SEQUENCING OF COMMANDS AND REPLIES

      The communication between the user and server is intended to be an
      alternating dialogue.  As such, the user issues an FTP command and
      the server responds with a prompt primary reply.  The user should
      wait for this initial primary success or failure response before
      sending further commands.

      Certain commands require a second reply for which the user should
      also wait.  These replies may, for example, report on the progress
      or completion of file transfer or the closing of the data
      connection.  They are secondary replies to file transfer commands.

      One important group of informational replies is the connection
      greetings.  Under normal circumstances, a server will send a 220
      reply, "awaiting input", when the connection is completed.  The
      user should wait for this greeting message before sending any
      commands.  If the server is unable to accept input right away, a
      120 "expected delay" reply should be sent immediately and a 220
      reply when ready.  The user will then know not to hang up if there
      is a delay.

      Spontaneous Replies

         Sometimes "the system" spontaneously has a message to be sent
         to a user (usually all users).  For example, "System going down
         in 15 minutes".  There is no provision in FTP for such
         spontaneous information to be sent from the server to the user.
         It is recommended that such information be queued in the
         server-PI and delivered to the user-PI in the next reply
         (possibly making it a multi-line reply).

      The table below lists alternative success and failure replies for
      each command.  These must be strictly adhered to; a server may
      substitute text in the replies, but the meaning and action implied
      by the code numbers and by the specific command reply sequence
      cannot be altered.

      Command-Reply Sequences

         In this section, the command-reply sequence is presented.  Each
         command is listed with its possible replies; command groups are
         listed together.  Preliminary replies are listed first (with
         their succeeding replies indented and under them), then
         positive and negative completion, and finally intermediary





         replies with the remaining commands from the sequence
         following.  This listing forms the basis for the state
         diagrams, which will be presented separately.

            Connection Establishment
               120
                  220
               220
               421
            Login
               USER
                  230
                  530
                  500, 501, 421
                  331, 332
               PASS
                  230
                  202
                  530
                  500, 501, 503, 421
                  332
               ACCT
                  230
                  202
                  530
                  500, 501, 503, 421
               CWD
                  250
                  500, 501, 502, 421, 530, 550
               CDUP
                  200
                  500, 501, 502, 421, 530, 550
               SMNT
                  202, 250
                  500, 501, 502, 421, 530, 550
            Logout
               REIN
                  120
                     220
                  220
                  421
                  500, 502
               QUIT
                  221
                  500





            Transfer parameters
               PORT
                  200
                  500, 501, 421, 530
               PASV
                  227
                  500, 501, 502, 421, 530
               MODE
                  200
                  500, 501, 504, 421, 530
               TYPE
                  200
                  500, 501, 504, 421, 530
               STRU
                  200
                  500, 501, 504, 421, 530
            File action commands
               ALLO
                  200
                  202
                  500, 501, 504, 421, 530
               REST
                  500, 501, 502, 421, 530
                  350
               STOR
                  125, 150
                     (110)
                     226, 250
                     425, 426, 451, 551, 552
                  532, 450, 452, 553
                  500, 501, 421, 530
               STOU
                  125, 150
                     (110)
                     226, 250
                     425, 426, 451, 551, 552
                  532, 450, 452, 553
                  500, 501, 421, 530
               RETR
                  125, 150
                     (110)
                     226, 250
                     425, 426, 451
                  450, 550
                  500, 501, 421, 530





               LIST
                  125, 150
                     226, 250
                     425, 426, 451
                  450
                  500, 501, 502, 421, 530
               NLST
                  125, 150
                     226, 250
                     425, 426, 451
                  450
                  500, 501, 502, 421, 530
               APPE
                  125, 150
                     (110)
                     226, 250
                     425, 426, 451, 551, 552
                  532, 450, 550, 452, 553
                  500, 501, 502, 421, 530
               RNFR
                  450, 550
                  500, 501, 502, 421, 530
                  350
               RNTO
                  250
                  532, 553
                  500, 501, 502, 503, 421, 530
               DELE
                  250
                  450, 550
                  500, 501, 502, 421, 530
               RMD
                  250
                  500, 501, 502, 421, 530, 550
               MKD
                  257
                  500, 501, 502, 421, 530, 550
               PWD
                  257
                  500, 501, 502, 421, 550
               ABOR
                  225, 226
                  500, 501, 502, 421







            Informational commands
               SYST
                  215
                  500, 501, 502, 421
               STAT
                  211, 212, 213
                  450
                  500, 501, 502, 421, 530
               HELP
                  211, 214
                  500, 501, 502, 421
            Miscellaneous commands
               SITE
                  200
                  202
                  500, 501, 530
               NOOP
                  200
                  500 421