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RFC 0765

File Transfer Protocol specification

Pages: 70
Obsoletes:  0542
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ToP   noToC   RFC0765 - Page 1
IEN 149                                                        J. Postel
RFC 765                                                              ISI
                                                               June 1980

                         FILE TRANSFER PROTOCOL


   The objectives of FTP are 1) to promote sharing of files (computer
   programs and/or data), 2) to encourage indirect or implicit (via
   programs) use of remote computers, 3) to shield a user from
   variations in file storage systems among Hosts, and 4) to transfer
   data reliably and efficiently.  FTP, though usable directly by a user
   at a terminal, is designed mainly for use by programs.

   The attempt in this specification is to satisfy the diverse needs of
   users of maxi-Hosts, mini-Hosts, and TIPs, with a simple, and easily
   implemented protocol design.

   This paper assumes knowledge of the following protocols described in
   the ARPA Internet Protocol Handbook.

      The Transmission Control Protocol

      The TELNET Protocol


   In this section, the terminology and the FTP model are discussed.
   The terms defined in this section are only those that have special
   significance in FTP.  Some of the terminology is very specific to the
   FTP model; some readers may wish to turn to the section on the FTP
   model while reviewing the terminology.



         The ASCII character set as defined in the ARPA Internet
         Protocol Handbook.  In FTP, ASCII characters are defined to be
         the lower half of an eight-bit code set (i.e., the most
         significant bit is zero).

      access controls

         Access controls define users' access privileges to the use of a
         system, and to the files in that system.  Access controls are
         necessary to prevent unauthorized or accidental use of files.
         It is the prerogative of a server-FTP process to invoke access
ToP   noToC   RFC0765 - Page 2
      byte size

         There are two byte sizes of interest in FTP:  the logical byte
         size of the file, and the transfer byte size used for the
         transmission of the data.  The transfer byte size is always 8
         bits.  The transfer byte size is not necessarily the byte size
         in which data is to be stored in a system, nor the logical byte
         size for interpretation of the structure of the data.

      data connection

         A simplex connection over which data is transferred, in a
         specified mode and type. The data transferred may be a part of
         a file, an entire file or a number of files.  The path may be
         between a server-DTP and a user-DTP, or between two

      data port

         The passive data transfer process "listens" on the data port
         for a connection from the active transfer process in order to
         open the data connection.


         The end-of-file condition that defines the end of a file being


         The end-of-record condition that defines the end of a record
         being transferred.

      error recovery

         A procedure that allows a user to recover from certain errors
         such as failure of either Host system or transfer process.  In
         FTP, error recovery may involve restarting a file transfer at a
         given checkpoint.

      FTP commands

         A set of commands that comprise the control information flowing
         from the user-FTP to the server-FTP process.
ToP   noToC   RFC0765 - Page 3

         An ordered set of computer data (including programs), of
         arbitrary length, uniquely identified by a pathname.


         The mode in which data is to be transferred via the data
         connection. The mode defines the data format during transfer
         including EOR and EOF.  The transfer modes defined in FTP are
         described in the Section on Transmission Modes.


         The Network Virtual Terminal as defined in the TELNET Protocol.


         The Network Virtual File System.  A concept which defines a
         standard network file system with standard commands and
         pathname conventions.  FTP only partially implements the NVFS
         concept at this time.


         A file may be structured as a set of independent parts called
         pages.  FTP supports the transmission of discontinuous files as
         independent indexed pages.


         Pathname is defined to be the character string which must be
         input to a file system by a user in order to identify a file.
         Pathname normally contains device and/or directory names, and
         file name specification.  FTP does not yet specify a standard
         pathname convention.  Each user must follow the file naming
         conventions of the file systems involved in the transfer.


         A sequential file may be structured as a number of contiguous
         parts called records.  Record structures are supported by FTP
         but a file need not have record structure.
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         A reply is an acknowledgment (positive or negative) sent from
         server to user via the TELNET connections in response to FTP
         commands.  The general form of a reply is a completion code
         (including error codes) followed by a text string.  The codes
         are for use by programs and the text is usually intended for
         human users.


         The data transfer process, in its normal "active" state,
         establishes the data connection with the "listening" data port,
         sets up parameters for transfer and storage, and transfers data
         on command from its PI.  The DTP can be placed in a "passive"
         state to listen for, rather than initiate a, connection on the
         data port.

      server-FTP process

         A process or set of processes which perform the function of
         file transfer in cooperation with a user-FTP process and,
         possibly, another server.  The functions consist of a protocol
         interpreter (PI) and a data transfer process (DTP).


         The protocol interpreter "listens" on Port L for a connection
         from a user-PI and establishes a TELNET communication
         connection.  It receives standard FTP commands from the
         user-PI, sends replies, and governs the server-DTP.

      TELNET connections

         The full-duplex communication path between a user-PI and a
         server-PI, operating according to the TELNET Protocol.


         The data representation type used for data transfer and
         storage.  Type implies certain transformations between the time
         of data storage and data transfer.  The representation types
         defined in FTP are described in the Section on Establishing
         Data Connections.
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         A human being or a process on behalf of a human being wishing
         to obtain file transfer service.  The human user may interact
         directly with a server-FTP process, but use of a user-FTP
         process is preferred since the protocol design is weighted
         towards automata.


         The data transfer process "listens" on the data port for a
         connection from a server-FTP process.  If two servers are
         transferring data between them, the user-DTP is inactive.

      user-FTP process

         A set of functions including a protocol interpreter, a data
         transfer process and a user interface which together perform
         the function of file transfer in cooperation with one or more
         server-FTP processes.  The user interface allows a local
         language to be used in the command-reply dialogue with the


         The protocol interpreter initiates the TELNET connection from
         its port U to the server-FTP process, initiates FTP commands,
         and governs the user-DTP if that process is part of the file
ToP   noToC   RFC0765 - Page 6

      With the above definitions in mind, the following model (shown in
      Figure 1) may be diagrammed for an FTP service.

                                            ||   User  ||    --------
                                            ||Interface|<--->| User |
                                            |\----:----/|    --------
                  ----------                |     V     |
                  |/------\|  FTP Commands  |/---------\|
                  ||Server|<---------------->|   User  ||
                  ||  PI  ||   FTP Replies  ||    PI   ||
                  |\--:---/|                |\----:----/|
                  |   V    |                |     V     |
      --------    |/------\|      Data      |/---------\|    --------
      | File |<--->|Server|<---------------->|  User   |<--->| File |
      |System|    || DTP  ||   Connection   ||   DTP   ||    |System|
      --------    |\------/|                |\---------/|    --------
                  ----------                -------------

                  Server-FTP                   User-FTP

      NOTES: 1. The data connection may be used in either direction.
             2. The data connection need not exist all of the time.

                      Figure 1  Model for FTP Use

      In the model described in Figure 1, the user-protocol interpreter
      initiates the TELNET connection. At the initiation of the user,
      standard FTP commands are generated by the user-PI and transmitted
      to the server process via the TELNET connection.  (The user may
      establish a direct TELNET connection to the server-FTP, from a TIP
      terminal for example, and generate standard FTP commands himself,
      bypassing the user-FTP process.) Standard replies are sent from
      the server-PI to the user-PI over the TELNET connection in
      response to the commands.

      The FTP commands specify the parameters for the data connection
      (data port, transfer mode, representation type, and structure) and
      the nature of file system operation (store, retrieve, append,
      delete, etc.).  The user-DTP or its designate should "listen" on
      the specified data port, and the server initiate the data
      connection and data transfer in accordance with the specified
      parameters.  It should be noted that the data port need not be in
ToP   noToC   RFC0765 - Page 7
      the same Host that initiates the FTP commands via the TELNET
      connection, but the user or his user-FTP process must ensure a
      "listen" on the specified data port.  It should also be noted that
      the data connection may be used for simultaneous sending and

      In another situation a user might wish to transfer files between
      two Hosts, neither of which is his local Host. He sets up TELNET
      connections to the two servers and then arranges for a data
      connection between them.  In this manner control information is
      passed to the user-PI but data is transferred between the server
      data transfer processes.  Following is a model of this
      server-server interaction.

                    TELNET     ------------    TELNET
                    ---------->| User-FTP |<-----------
                    |          | User-PI  |           |
                    |          |   "C"    |           |
                    V          ------------           V
            --------------                        --------------
            | Server-FTP |   Data Connection      | Server-FTP |
            |    "A"     |<---------------------->|    "B"     |
            --------------  Port (A)     Port (B) --------------

                                 Figure 2

      The protocol requires that the TELNET connections be open while
      data transfer is in progress.  It is the responsibility of the
      user to request the closing of the TELNET connections when
      finished using the FTP service, while it is the server who takes
      the action.  The server may abort data transfer if the TELNET
      connections are closed without command.


   Files are transferred only via the data connection.  The TELNET
   connection is used for the transfer of commands, which describe the
   functions to be performed, and the replies to these commands (see the
   Section on FTP Replies).  Several commands are concerned with the
   transfer of data between Hosts.  These data transfer commands include
   the MODE command which specify how the bits of the data are to be
   transmitted, and the STRUcture and TYPE commands, which are used to
   define the way in which the data are to be represented. The
   transmission and representation are basically independent but
ToP   noToC   RFC0765 - Page 8
   "Stream" transmission mode is dependent on the file structure
   attribute and if "Compressed" transmission mode is used the nature of
   the filler byte depends on the representation type.


      Data is transferred from a storage device in the sending Host to a
      storage device in the receiving Host.  Often it is necessary to
      perform certain transformations on the data because data storage
      representations in the two systems are different.  For example,
      NVT-ASCII has different data storage representations in different
      systems.  PDP-10's generally store NVT-ASCII as five 7-bit ASCII
      characters, left-justified in a 36-bit word. 360's store NVT-ASCII
      as 8-bit EBCDIC codes. Multics stores NVT-ASCII as four 9-bit
      characters in a 36-bit word.  It may be desirable to convert
      characters into the standard NVT-ASCII representation when
      transmitting text between dissimilar systems.  The sending and
      receiving sites would have to perform the necessary
      transformations between the standard representation and their
      internal representations.

      A different problem in representation arises when transmitting
      binary data (not character codes) between Host systems with
      different word lengths.  It is not always clear how the sender
      should send data, and the receiver store it.  For example, when
      transmitting 32-bit bytes from a 32-bit word-length system to a
      36-bit word-length system, it may be desirable (for reasons of
      efficiency and usefulness) to store the 32-bit bytes
      right-justified in a 36-bit word in the latter system.  In any
      case, the user should have the option of specifying data
      representation and transformation functions.  It should be noted
      that FTP provides for very limited data type representations.
      Transformations desired beyond this limited capability should be
      performed by the user directly.

      Data representations are handled in FTP by a user specifying a
      representation type.  This type may implicitly (as in ASCII or
      EBCDIC) or explicitly (as in Local byte) define a byte size for
      interpretation which is referred to as the "logical byte size."
      This has nothing to do with the byte size used for transmission
      over the data connection, called the "transfer byte size", and the
      two should not be confused.  For example, NVT-ASCII has a logical
      byte size of 8 bits.  If the type is Local byte, then the TYPE
      command has an obligatory second parameter specifying the logical
      byte size.  The transfer byte size is always 8 bits.
ToP   noToC   RFC0765 - Page 9
      The types ASCII and EBCDIC also take a second (optional)
      parameter; this is to indicate what kind of vertical format
      control, if any, is associated with a file.  The following data
      representation types are defined in FTP:

         ASCII Format

            This is the default type and must be accepted by all FTP
            implementations.  It is intended primarily for the transfer
            of text files, except when both Hosts would find the EBCDIC
            type more convenient.

            The sender converts the data from his internal character
            representation to the standard 8-bit NVT-ASCII
            representation (see the TELNET specification).  The receiver
            will convert the data from the standard form to his own
            internal form.

            In accordance with the NVT standard, the <CRLF> sequence
            should be used, where necessary, to denote the end of a line
            of text.  (See the discussion of file structure at the end
            of the Section on Data Representation and Storage).

            Using the standard NVT-ASCII representation means that data
            must be interpreted as 8-bit bytes.

            The Format parameter for ASCII and EBCDIC types is discussed

         EBCDIC Format

            This type is intended for efficient transfer between Hosts
            which use EBCDIC for their internal character

            For transmission the data are represented as 8-bit EBCDIC
            characters.  The character code is the only difference
            between the functional specifications of EBCDIC and ASCII

            End-of-line (as opposed to end-of-record--see the discussion
            of structure) will probably be rarely used with EBCDIC type
            for purposes of denoting structure, but where it is
            necessary the <NL> character should be used.
ToP   noToC   RFC0765 - Page 10
      A character file may be transferred to a Host for one of three
      purposes: for printing, for storage and later retrieval, or for
      processing.  If a file is sent for printing, the receiving Host
      must know how the vertical format control is represented.  In the
      second case, it must be possible to store a file at a Host and
      then retrieve it later in exactly the same form.  Finally, it
      ought to be possible to move a file from one Host to another and
      process the file at the second Host without undue trouble.  A
      single ASCII or EBCDIC format does not satisfy all these
      conditions and so these types have a second parameter specifying
      one of the following three formats:


            This is the default format to be used if the second (format)
            parameter is omitted.  Non-print format must be accepted by
            all FTP implementations.

            The file need contain no vertical format information.  If it
            is passed to a printer process, this process may assume
            standard values for spacing and margins.

            Normally, this format will be used with files destined for
            processing or just storage.

         TELNET Format Controls

            The file contains ASCII/EBCDIC vertical format controls
            (i.e., <CR>, <LF>, <NL>, <VT>, <FF>) which the printer
            process will interpret appropriately.  <CRLF>, in exactly
            this sequence, also denotes end-of-line.

         Carriage Control (ASA)

            The file contains ASA (FORTRAN) vertical format control
            characters.  (See RFC 740 Appendix C and Communications of
            the ACM, Vol. 7, No. 10, 606 (Oct. 1964)).  In a line or a
            record, formatted according to the ASA Standard, the first
            character is not to be printed.  Instead it should be used
            to determine the vertical movement of the paper which should
            take place before the rest of the record is printed.
ToP   noToC   RFC0765 - Page 11
            The ASA Standard specifies the following control characters:

               Character     Vertical Spacing

               blank         Move paper up one line
               0             Move paper up two lines
               1             Move paper to top of next page
               +             No movement, i.e., overprint

            Clearly there must be some way for a printer process to
            distinguish the end of the structural entity.  If a file has
            record structure (see below) this is no problem; records
            will be explicitly marked during transfer and storage.  If
            the file has no record structure, the <CRLF> end-of-line
            sequence is used to separate printing lines, but these
            format effectors are overridden by the ASA controls.


            The data are sent as contiguous bits which, for transfer,
            are packed into the 8-bit transfer bytes.  The receiving
            site must store the data as contiguous bits.  The structure
            of the storage system might necessitate the padding of the
            file (or of each record, for a record-structured file) to
            some convenient boundary (byte, word or block).  This
            padding, which must be all zeros, may occur only at the end
            of the file (or at the end of each record) and there must be
            a way of identifying the padding bits so that they may be
            stripped off if the file is retrieved.  The padding
            transformation should be well publicized to enable a user to
            process a file at the storage site.

            Image type is intended for the efficient storage and
            retrieval of files and for the transfer of binary data.  It
            is recommended that this type be accepted by all FTP

         Local byte Byte size

            The data is transferred in logical bytes of the size
            specified by the obligatory second parameter, Byte size.
            The value of Byte size must be a decimal integer; there is
            no default value.  The logical byte size is not necessarily
            the same as the transfer byte size.  If there is a
ToP   noToC   RFC0765 - Page 12
            difference in byte sizes, then the logical bytes should be
            packed contiguously, disregarding transfer byte boundaries
            and with any necessary padding at the end.

            When the data reaches the receiving Host it will be
            transformed in a manner dependent on the logical byte size
            and the particular Host.  This transformation must be
            invertible (that is an identical file can be retrieved if
            the same parameters are used) and should be well publicized
            by the FTP implementors.

            For example, a user sending 36-bit floating-point numbers to
            a Host with a 32-bit word could send his data as Local byte
            with a logical byte size of 36.  The receiving Host would
            then be expected to store the logical bytes so that they
            could be easily manipulated; in this example putting the
            36-bit logical bytes into 64-bit double words should

            Another example, a pair of hosts with a 36-bit word size may
            send data to one another in words by using TYPE L 36.  The
            data would be sent in the 8-bit transmission bytes packed so
            that 9 transmission bytes carried two host words.

      A note of caution about parameters:  a file must be stored and
      retrieved with the same parameters if the retrieved version is to
      be identical to the version originally transmitted.  Conversely,
      FTP implementations must return a file identical to the original
      if the parameters used to store and retrieve a file are the same.

      In addition to different representation types, FTP allows the
      structure of a file to be specified.  Three file structures are
      defined in FTP:

         file-structure, where there is no internal structure and the
                           file is considered to be a continuous
                           sequence of data bytes,

         record-structure, where the file is made up of sequential

         and page-structure, where the file is made up of independent
                           indexed pages.

      File-structure is the default, to be assumed if the STRUcture
      command has not been used but both file and record structures must
ToP   noToC   RFC0765 - Page 13
      be accepted for "text" files (i.e., files with TYPE ASCII or
      EBCDIC) by all FTP implementations.  The structure of a file will
      affect both the transfer mode of a file (see the Section on
      Transmission Modes) and the interpretation and storage of the

      The "natural" structure of a file will depend on which Host stores
      the file.  A source-code file will usually be stored on an IBM 360
      in fixed length records but on a PDP-10 as a stream of characters
      partitioned into lines, for example by <CRLF>.  If the transfer of
      files between such disparate sites is to be useful, there must be
      some way for one site to recognize the other's assumptions about
      the file.

      With some sites being naturally file-oriented and others naturally
      record-oriented there may be problems if a file with one structure
      is sent to a Host oriented to the other.  If a text file is sent
      with record-structure to a Host which is file oriented, then that
      Host should apply an internal transformation to the file based on
      the record structure.  Obviously this transformation should be
      useful but it must also be invertible so that an identical file
      may be retrieved using record structure.

      In the case of a file being sent with file-structure to a
      record-oriented Host, there exists the question of what criteria
      the Host should use to divide the file into records which can be
      processed locally.  If this division is necessary the FTP
      implementation should use the end-of-line sequence, <CRLF> for
      ASCII, or <NL> for EBCDIC text files, as the delimiter.  If an FTP
      implementation adopts this technique, it must be prepared to
      reverse the transformation if the file is retrieved with

      Page Structure

         To transmit files that are discontinuous FTP defines a page
         structure.  Files of this type are sometimes know as "random
         access files" or even as "holey files".  In these files there
         is sometimes other information associated with the file as a
         whole (e.g., a file descriptor), or with a section of the file
         (e.g., page access controls), or both.  In FTP, the sections of
         the file are called pages.

         To provide for various page sizes and associated information
         each page is sent with a page header.  The page header has the
         following defined fields:
ToP   noToC   RFC0765 - Page 14
            Header Length

               The number of logical bytes in the page header including
               this byte.  The minimum header length is 4.

            Page Index

               The logical page number of this section of the file.
               This is not the transmission sequence number of this
               page, but the index used to identify this page of the

            Data Length

               The number of logical bytes in the page data.  The
               minimum data length is 0.

            Page Type

               The type of page this is.  The following page types are

                  0 = Last Page

                     This is used to indicate the end of a paged
                     structured transmission.  The header length must be
                     4, and the data length must be 0.

                  1 = Simple Page

                     This is the normal type for simple paged files with
                     no page level associated control information.  The
                     header length must be 4.

                  2 = Descriptor Page

                     This type is used to transmit the descriptive
                     information for the file as a whole.

                  3 = Access Controled Page

                     This is type includes an additional header field
                     for paged files with page level access control
                     information.  The header length must be 5.
ToP   noToC   RFC0765 - Page 15
            Optional Fields

               Further header fields may be used to supply per page
               control information, for example, per page access

         All fields are one logical byte in length.  The logical byte
         size is specified by the TYPE command.


      The mechanics of transferring data consists of setting up the data
      connection to the appropriate ports and choosing the parameters
      for transfer.  Both the user and the server-DTPs have a default
      data port.  The user-process default data port is the same as the
      control connection port, i.e., U.  The server-process default data
      port is the port adjacent to the control connection port, i.e.,

      The transfer byte size is 8-bit bytes.  This byte size is relevant
      only for the actual transfer of the data; it has no bearing on
      representation of the data within a Host's file system.

      The passive data transfer process (this may be a user-DTP or a
      second server-DTP) shall "listen" on the data port prior to
      sending a transfer request command.  The FTP request command
      determines the direction of the data transfer.  The server, upon
      receiving the transfer request, will initiate the data connection
      to the port.  When the connection is established, the data
      transfer begins between DTP's, and the server-PI sends a
      confirming reply to the user-PI.

      It is possible for the user to specify an alternate data port by
      use of the PORT command.  He might want a file dumped on a TIP
      line printer or retrieved from a third party Host.  In the latter
      case the user-PI sets up TELNET connections with both server-PI's.
      One server is then told (by an FTP command) to "listen" for a
      connection which the other will initiate.  The user-PI sends one
      server-PI a PORT command indicating the data port of the other.
      Finally both are sent the appropriate transfer commands.  The
      exact sequence of commands and replies sent between the
      user-controller and the servers is defined in the Section on FTP

      In general it is the server's responsibility to maintain the data
      connection--to initiate it and to close it.  The exception to this
ToP   noToC   RFC0765 - Page 16
      is when the user-DTP is sending the data in a transfer mode that
      requires the connection to be closed to indicate EOF.  The server
      MUST close the data connection under the following conditions:

         1. The server has completed sending data in a transfer mode
            that requires a close to indicate EOF.

         2. The server receives an ABORT command from the user.

         3. The port specification is changed by a command from the

         4. The TELNET connection is closed legally or otherwise.

         5. An irrecoverable error condition occurs.

      Otherwise the close is a server option, the exercise of which he
      must indicate to the user-process by an appropriate reply.


      The next consideration in transferring data is choosing the
      appropriate transmission mode.  There are three modes: one which
      formats the data and allows for restart procedures; one which also
      compresses the data for efficient transfer; and one which passes
      the data with little or no processing.  In this last case the mode
      interacts with the structure attribute to determine the type of
      processing.  In the compressed mode the representation type
      determines the filler byte.

      All data transfers must be completed with an end-of-file (EOF)
      which may be explicitly stated or implied by the closing of the
      data connection.  For files with record structure, all the
      end-of-record markers (EOR) are explicit, including the final one.
      For files transmitted in page structure a "last-page" page type is

      NOTE:  In the rest of this section, byte means "transfer byte"
      except where explicitly stated otherwise.

      For the purpose of standardized transfer, the sending Host will
      translate his internal end of line or end of record denotation
      into the representation prescribed by the transfer mode and file
      structure, and the receiving Host will perform the inverse
      translation to his internal denotation.  An IBM 360 record count
      field may not be recognized at another Host, so the end of record
ToP   noToC   RFC0765 - Page 17
      information may be transferred as a two byte control code in
      Stream mode or as a flagged bit in a Block or Compressed mode
      descriptor. End of line in an ASCII or EBCDIC file with no record
      structure should be indicated by <CRLF> or <NL>, respectively.
      Since these transformations imply extra work for some systems,
      identical systems transferring non-record structured text files
      might wish to use a binary representation and stream mode for the

      The following transmission modes are defined in FTP:


            The data is transmitted as a stream of bytes.  There is no
            restriction on the representation type used; record
            structures are allowed.

            In a record structured file EOR and EOF will each be
            indicated by a two-byte control code.  The first byte of the
            control code will be all ones, the escape character.  The
            second byte will have the low order bit on and zeros
            elsewhere for EOR and the second low order bit on for EOF;
            that is, the byte will have value 1 for EOR and value 2 for
            EOF.  EOR and EOF may be indicated together on the last byte
            transmitted by turning both low order bits on, i.e., the
            value 3.  If a byte of all ones was intended to be sent as
            data, it should be repeated in the second byte of the
            control code.

            If the structure is file structure, the EOF is indicated by
            the sending Host closing the data connection and all bytes
            are data bytes.


            The file is transmitted as a series of data blocks preceded
            by one or more header bytes.  The header bytes contain a
            count field, and descriptor code.  The count field indicates
            the total length of the data block in bytes, thus marking
            the beginning of the next data block (there are no filler
            bits). The descriptor code defines:  last block in the file
            (EOF) last block in the record (EOR), restart marker (see
            the Section on Error Recovery and Restart) or suspect data
            (i.e., the data being transferred is suspected of errors and
            is not reliable).  This last code is NOT intended for error
            control within FTP.  It is motivated by the desire of sites
ToP   noToC   RFC0765 - Page 18
            exchanging certain types of data (e.g., seismic or weather
            data) to send and receive all the data despite local errors
            (such as "magnetic tape read errors"), but to indicate in
            the transmission that certain portions are suspect).  Record
            structures are allowed in this mode, and any representation
            type may be used.

            The header consists of the three bytes.  Of the 24 bits of
            header information, the 16 low order bits shall represent
            byte count, and the 8 high order bits shall represent
            descriptor codes as shown below.

            Block Header

               | Descriptor     |    Byte Count                   |
               |         8 bits |                      16 bits    |

            The descriptor codes are indicated by bit flags in the
            descriptor byte.  Four codes have been assigned, where each
            code number is the decimal value of the corresponding bit in
            the byte.

               Code     Meaning
                128     End of data block is EOR
                 64     End of data block is EOF
                 32     Suspected errors in data block
                 16     Data block is a restart marker


            With this encoding more than one descriptor coded condition
            may exist for a particular block.  As many bits as necessary
            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 TELNET
            connection (e.g., default--NVT-ASCII).  <SP> (Space, in the
            appropriate language) must not be used WITHIN a restart
ToP   noToC   RFC0765 - Page 19
            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 |


            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:

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