rfc959.txt

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         may be flagged.

         The restart marker is embedded in the data stream as an
         integral number of 8-bit bytes representing printable
         characters in the language being used over the control
         connection (e.g., default--NVT-ASCII).  <SP> (Space, in the
         appropriate language) must not be used WITHIN a restart marker.

         For example, to transmit a six-character marker, the following
         would be sent:

            +--------+--------+--------+
            |Descrptr|  Byte count     |
            |code= 16|             = 6 |
            +--------+--------+--------+

            +--------+--------+--------+
            | Marker | Marker | Marker |
            | 8 bits | 8 bits | 8 bits |
            +--------+--------+--------+

            +--------+--------+--------+
            | Marker | Marker | Marker |
            | 8 bits | 8 bits | 8 bits |
            +--------+--------+--------+


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RFC 959                                                     October 1985
File Transfer Protocol


      3.4.3.  COMPRESSED MODE

         There are three kinds of information to be sent:  regular data,
         sent in a byte string; compressed data, consisting of
         replications or filler; and control information, sent in a
         two-byte escape sequence.  If n>0 bytes (up to 127) of regular
         data are sent, these n bytes are preceded by a byte with the
         left-most bit set to 0 and the right-most 7 bits containing the
         number n.

         Byte string:

             1       7                8                     8
            +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+
            |0|       n     | |    d(1)       | ... |      d(n)     |
            +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+
                                          ^             ^
                                          |---n bytes---|
                                              of data

            String of n data bytes d(1),..., d(n)
            Count n must be positive.

         To compress a string of n replications of the data byte d, the
         following 2 bytes are sent:

         Replicated Byte:

              2       6               8
            +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
            |1 0|     n     | |       d       |
            +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+

         A string of n filler bytes can be compressed into a single
         byte, where the filler byte varies with the representation
         type.  If the type is ASCII or EBCDIC the filler byte is <SP>
         (Space, ASCII code 32, EBCDIC code 64).  If the type is Image
         or Local byte the filler is a zero byte.

         Filler String:

              2       6
            +-+-+-+-+-+-+-+-+
            |1 1|     n     |
            +-+-+-+-+-+-+-+-+

         The escape sequence is a double byte, the first of which is the


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File Transfer Protocol


         escape byte (all zeros) and the second of which contains
         descriptor codes as defined in Block mode.  The descriptor
         codes have the same meaning as in Block mode and apply to the
         succeeding string of bytes.

         Compressed mode is useful for obtaining increased bandwidth on
         very large network transmissions at a little extra CPU cost.
         It can be most effectively used to reduce the size of printer
         files such as those generated by RJE hosts.

   3.5.  ERROR RECOVERY AND RESTART

      There is no provision for detecting bits lost or scrambled in data
      transfer; this level of error control is handled by the TCP.
      However, a restart procedure is provided to protect users from
      gross system failures (including failures of a host, an
      FTP-process, or the underlying network).

      The restart procedure is defined only for the block and compressed
      modes of data transfer.  It requires the sender of data to insert
      a special marker code in the data stream with some marker
      information.  The marker information has meaning only to the
      sender, but must consist of printable characters in the default or
      negotiated language of the control connection (ASCII or EBCDIC).
      The marker could represent a bit-count, a record-count, or any
      other information by which a system may identify a data
      checkpoint.  The receiver of data, if it implements the restart
      procedure, would then mark the corresponding position of this
      marker in the receiving system, and return this information to the
      user.

      In the event of a system failure, the user can restart the data
      transfer by identifying the marker point with the FTP restart
      procedure.  The following example illustrates the use of the
      restart procedure.

      The sender of the data inserts an appropriate marker block in the
      data stream at a convenient point.  The receiving host marks the
      corresponding data point in its file system and conveys the last
      known sender and receiver marker information to the user, either
      directly or over the control connection in a 110 reply (depending
      on who is the sender).  In the event of a system failure, the user
      or controller process restarts the server at the last server
      marker by sending a restart command with server's marker code as
      its argument.  The restart command is transmitted over the control




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RFC 959                                                     October 1985
File Transfer Protocol


      connection and is immediately followed by the command (such as
      RETR, STOR or LIST) which was being executed when the system
      failure occurred.

4.  FILE TRANSFER FUNCTIONS

   The communication channel from the user-PI to the server-PI is
   established as a TCP connection from the user to the standard server
   port.  The user protocol interpreter is responsible for sending FTP
   commands and interpreting the replies received; the server-PI
   interprets commands, sends replies and directs its DTP to set up the
   data connection and transfer the data.  If the second party to the
   data transfer (the passive transfer process) is the user-DTP, then it
   is governed through the internal protocol of the user-FTP host; if it
   is a second server-DTP, then it is governed by its PI on command from
   the user-PI.  The FTP replies are discussed in the next section.  In
   the description of a few of the commands in this section, it is
   helpful to be explicit about the possible replies.

   4.1.  FTP COMMANDS

      4.1.1.  ACCESS CONTROL COMMANDS

         The following commands specify access control identifiers
         (command codes are shown in parentheses).

         USER NAME (USER)

            The argument field is a Telnet string identifying the user.
            The user identification is that which is required by the
            server for access to its file system.  This command will
            normally be the first command transmitted by the user after
            the control connections are made (some servers may require
            this).  Additional identification information in the form of
            a password and/or an account command may also be required by
            some servers.  Servers may allow a new USER command to be
            entered at any point in order to change the access control
            and/or accounting information.  This has the effect of
            flushing any user, password, and account information already
            supplied and beginning the login sequence again.  All
            transfer parameters are unchanged and any file transfer in
            progress is completed under the old access control
            parameters.






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File Transfer Protocol


         PASSWORD (PASS)

            The argument field is a Telnet string specifying the user's
            password.  This command must be immediately preceded by the
            user name command, and, for some sites, completes the user's
            identification for access control.  Since password
            information is quite sensitive, it is desirable in general
            to "mask" it or suppress typeout.  It appears that the
            server has no foolproof way to achieve this.  It is
            therefore the responsibility of the user-FTP process to hide
            the sensitive password information.

         ACCOUNT (ACCT)

            The argument field is a Telnet string identifying the user's
            account.  The command is not necessarily related to the USER
            command, as some sites may require an account for login and
            others only for specific access, such as storing files.  In
            the latter case the command may arrive at any time.

            There are reply codes to differentiate these cases for the
            automation: when account information is required for login,
            the response to a successful PASSword command is reply code
            332.  On the other hand, if account information is NOT
            required for login, the reply to a successful PASSword
            command is 230; and if the account information is needed for
            a command issued later in the dialogue, the server should
            return a 332 or 532 reply depending on whether it stores
            (pending receipt of the ACCounT command) or discards the
            command, respectively.

         CHANGE WORKING DIRECTORY (CWD)

            This command allows the user to work with a different
            directory or dataset for file storage or retrieval without
            altering his login or accounting information.  Transfer
            parameters are similarly unchanged.  The argument is a
            pathname specifying a directory or other system dependent
            file group designator.

         CHANGE TO PARENT DIRECTORY (CDUP)

            This command is a special case of CWD, and is included to
            simplify the implementation of programs for transferring
            directory trees between operating systems having different




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File Transfer Protocol


            syntaxes for naming the parent directory.  The reply codes
            shall be identical to the reply codes of CWD.  See
            Appendix II for further details.

         STRUCTURE MOUNT (SMNT)

            This command allows the user to mount a different file
            system data structure without altering his login or
            accounting information.  Transfer parameters are similarly
            unchanged.  The argument is a pathname specifying a
            directory or other system dependent file group designator.

         REINITIALIZE (REIN)

            This command terminates a USER, flushing all I/O and account
            information, except to allow any transfer in progress to be
            completed.  All parameters are reset to the default settings
            and the control connection is left open.  This is identical
            to the state in which a user finds himself immediately after
            the control connection is opened.  A USER command may be
            expected to follow.

         LOGOUT (QUIT)

            This command terminates a USER and if file transfer is not
            in progress, the server closes the control connection.  If
            file transfer is in progress, the connection will remain
            open for result response and the server will then close it.
            If the user-process is transferring files for several USERs
            but does not wish to close and then reopen connections for
            each, then the REIN command should be used instead of QUIT.

            An unexpected close on the control connection will cause the
            server to take the effective action of an abort (ABOR) and a
            logout (QUIT).

      4.1.2.  TRANSFER PARAMETER COMMANDS

         All data transfer parameters have default values, and the
         commands specifying data transfer parameters are required only
         if the default parameter values are to be changed.  The default
         value is the last specified value, or if no value has been
         specified, the standard default value