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Network Working Group Abhay Bhushan Request for Comments: 171 MIT NIC 6793 Bob Braden Categories: D.4, D.5, and D.7 UCLA Updates: 114 Will Crowther Obsolete: None Alex McKenzie

                                                          Eric Harslem
                                                          John Heafner
                                                           John Melvin
                                                           Dick Watson
                                                          Bob Sundberg
                                                             Jim White
                                                          23 June 1971
                     THE DATA TRANSFER PROTOCOL


 A common protocol is desirable for data transfer in such diverse
 applications as remote job entry, file transfer, network mail system,
 graphics, remote program execution, and communication with block data
 terminals (such as printers, card, paper tape, and magnetic tape
 equipment, especially in context of terminal IMPs).  Although it
 would be possible to include some or even all of the above
 applications in an all-inclusive file transfer protocol, a separation
 between data transfer and application functions would provide
 flexibility in implementation, and reduce complexity.  Separating the
 data transfer function would also reduce proliferation of programs
 and protocols.
 We have therefore defined a low-level data transfer protocol (DTP) to
 be used for transfer of data in file transfer, remote job entry, and
 other applications protocols.  This paper concerns itself solely with
 the data transfer protocol.  A companion paper (RFC 172) describes
 file transfer protocol.


 The data transfer protocol (DTP) serves three basic functions.  It
 provides for convenient separation of NCP messages into "logical"
 blocks (transactions, units, records, groups, and files), it allows
 for the separation of data and control information, and it includes
 some error control mechanisms.

Bhushan, et al. [Page 1] RFC 171 THE DATA TRANSFER PROTOCOL June 1971

 Three modes of separating messages into transactions [1] are allowed
 by DTP.  The first is an indefinite bit stream which terminates only
 when the connection is closed (i.e., the bit stream represents a
 single transaction for duration of connection).  This mode would be
 useful in data transfer between hosts and terminal IMPs (TIPs).
 The second mode utilizes a "transparent" block convention, similar to
 the ASCII DLE (Data Link Escape).  In "transparent" mode,
 transactions (which may be arbitrarily long) end whenever the
 character sequence DLE ETX is encountered (DLE and ETX are 8-bit
 character codes).  To prevent the possibility of a DLE ETX sequence
 occurring within data stream, any occurrence of DLE is replaced by
 DLE DLE on transmission.  The extra DLE is stripped on reception.  A
 departure from the ASCII convention is that "transparent" block does
 not begin with DLE STX, but with a transaction type byte.  This mode
 will be useful in data transfer between terminal IMPs.
 The third mode utilizes a count mechanism.  Each transaction begins
 with a fixed-length descriptor field containing separate binary
 counts of information bits and filler bits.  If a transaction has no
 filler bits, its filler count is zero.  This mode will be useful in
 most host-to-host data transfer applications.
 DTP allows for the above modes to be intermixed over the same
 connection (i.e., mode is not associated with connection, but only
 with transaction).  The above transfer modes can represent transfer
 of either data or control information.  The protocol allows for
 separating data or control information at a lower level, by providing
 different "type" codes (see SPECIFICATIONS) for data and control
 transactions.  This provision may simplify some implementations.
 The implementation of a workable [2] subset of the above modes is
 specifically permitted by DTP.  To provide compatibility between
 hosts using different subsets of transfer modes, an initial
 "handshake" procedure is required by DTP.  The handshake involves
 exchanging information on modes available for transmit and receive.
 This will enable host programs to agree on transfer modes acceptable
 for a connection.
 The manner in which DTP is used would depend largely on the
 applications protocol.  It is the applications protocol which defines
 the workable subset of transfer modes.  For example, the file
 transfer protocol will not work just with the indefinite bit stream
 modes.  At least, for control information one of the other two modes
 is required.  Again, the use of information separator and abort
 functions provided in DTP (see SPECIFICATIONS) is defined by the
 applications protocol.  For example, in a remote job entry protocol,
 aborts may be used to stop the execution of a job while they may not

Bhushan, et al. [Page 2] RFC 171 THE DATA TRANSFER PROTOCOL June 1971

 cause any action in another applications protocol.
 It should also be noted that DTP does not define a data transfer
 service.  There is no standard server socket, or initial connection
 protocol defined for DTP.  What DTP defines is a mechanism for data
 transfer which can be used to provide services for block data
 transfers, file transfers, remote job entry, network mail and
 numerous other applications.
 There are to be no restrictions on the manner in which DTP is
 implemented at various sites.  For example, DTP may be imbedded in an
 applications program such as for file transfer, or it may be a
 separate service program or subroutine used by several applications
 programs.  Another implementation may employ macros or UUO's (user
 unimplemented operations on PDP-10's), to achieve the functions
 specified in DTP.  It is also possible that in implementation, the
 separation between the DTP and applications protocols be only at a
 conceptual level.


 1.  Byte Size for Network Connection
     The standard byte size for network connections using DTP is 8-
     bit.  However, other byte sizes specified by higher-level
     applications protocols or applications programs are also allowed
     by DTP.  For the purpose of this document bytes are assumed to be
     8-bits, unless otherwise stated.

2. Transactions

     At DTP level, all information transmitted over connection is a
     sequence of transactions.  DTP defines the rules for delimiting
     transactions. [3]

2A. Types

     The first byte of each transaction shall define a transaction
     type, as shown below.  (Note that code assignments do not
     conflict with assignments in TELNET protocol.)  The transaction
     types may be referred by the hexadecimal code assigned to them.
     The transactions types are discussed in more detail in section

Bhushan, et al. [Page 3] RFC 171 THE DATA TRANSFER PROTOCOL June 1971

         Code                 Transaction Type
    Hex       Octal
    B0         260        Indefinite bit stream -- data.
    B1         261        Transparent (DLE) block--data.
    B2         262        Descriptor and counts--data.
    B3         263        Modes available (handshake).
    B4         264        Information separators (endcode).
    B5         265        Error codes.
    B6         266        Abort.
    B7         267        No operation (NoOp).
    B8         270        Indefinite bit stream--control.
    B9         271        Transparent (DLE) block--control.
    BA         272        Descriptor and counts--control.
    BB         273        (unassigned but reserved for data transfer)
    BC         274                  "         "         "
    BD         275                  "         "         "
    BE         276                  "         "         "
    BF         277                  "         "         "
 2B.  Syntax and Semantics
 2B.1  Type B0 and B8 (indefinite bitstream modes) transactions
       terminate only when the NCP connection is "closed".  There is
       no other escape convention defined in DTP at this level.  It
       should be noted, that closing connection in bitstream mode
       represents an implicit file separator (see section 2B.5).
 2B.2  Type B1 and B0 (transparent block modes) transactions terminate
       when the byte sequence DLE ETX is encountered.  The sender
       shall replace any occurrence of DLE in data stream by the
       sequence DLE DLE.  The receiver shall strip the extra DLE.  The
       transaction is assumed to by byte-oriented.  The code for DLE
       is Hex '90' or Octal '220' (this is different from the ASCII
       DLE which is Hex '10' or Octal '020).  ETX is Hex '03' or Octal
       '03' (the same as ASCII ETX) [4].
 2B.3  Type B2 and BA (descriptor and counts modes) transactions have
       three fields, a 9-byte (72-bits) descriptor field [5] and
       variable length (including zero) info and filler fields, as
       shown below.  The total length of a transaction is
       (72+info+filler) bits.

Bhushan, et al. [Page 4] RFC 171 THE DATA TRANSFER PROTOCOL June 1971

<B2 or BA><Info count><NUL><Seq #><NUL><filler count><info><filler>
3-bits 24-bits 8-bits 16-bits 8-bits 8-bits Variable length
←—- 72-bit descriptor field ———————>info and filler
       Info count is a binary count of number of bits in info field,
       not including descriptor or filler bits.  Number of info bits
       is limited to (2**24 - 1), as there are 24 bits in info count
       Sequence # is a sequential count in round-robin manner of B2
       and BA type transaction.  The inclusion of sequence numbers
       would help in debugging and error control, as sequence numbers
       may be used to check for missing transactions, and aid in
       locating errors.  Hosts not wishing to implement this mechanism
       should have all 1's in the field.  The count shall start from
       zero and continue sequentially to all 1's, after which it is
       reset to all zeros.  The permitted sequence numbers are one
       greater than the previous, and all 1's.
       Filler count is a binary count of bits used as fillers (i.e.,
       not information) after the end of meaningful data.  Number of
       filler bits is limited to 255, as there are 8 bits in filler
       count field.
       The NUL bytes contain all 0's.
 2B.4  Type B3 (modes available) transactions have a fixed length of 3
       bytes, as shown below.  First byte defines transaction type as
       B3, second byte defines modes available for send, and third
       byte defines modes available for receive.
       |    Type          |     I send          |     I receive       |
       |                  | | |  |  |  |  |  |  | | |  |  |  |  |  |  |
       |     B3           |0|0|BA|B2|B9|B1|B8|B0|0|0|BA|B2|B9|B1|B8|B0|
       The modes are indicated by bit-coding, as shown above.  The
       particular bit or bits, if set to logical "1", indicate that
       mode to be available.  The 2 most significant bits should be
       set to logical "0".  The use of type B3 transactions is
       discussed in section 3B.
 2B.5  Type B4 (information separator) transactions have fixed length
       of 2 bytes, as shown below.  First byte defines transaction
       type as B4, and second byte defines the separator.

Bhushan, et al. [Page 5] RFC 171 THE DATA TRANSFER PROTOCOL June 1971

       |    Type          |     End Code     |
       |                  |            | |R| |
       |                  |            |G|E| |
       |     B4           |           F|R|C|U|
       |                  |           I|O|O|N|
       |                  |           L|U|R|I|
       |                  |           E|P|D|T|
       The following separator codes are assigned:
                  Code                    Meaning
          Hex             Octal
          01              001             Unit separator
          03              003             Record separator
          07              007             Group separator
          0F              017             File separator
       Files, groups, records, and units may be data blocks that a
       user defines to be so.  The only restriction is that of the
       hierarchical relationship  File>Groups>Records>Units  (where
       '>' means 'contains').  Thus a file separator marks not only
       the end of file, but also the end of group, record, and unit.
       These separators may provide a convenient "logical" separation
       of data at the data transfer level.  Their use is governed by
       the applications protocol.
 2B.6  Type B5 (error codes) transactions have a fixed length of 3
       bytes, as shown below.  First byte defines transaction type as
       B5, second byte indicates an error code, and third byte may
       indicate the sequence number on which error occurred.
       |    Type          |     Error Code    |     Sequence #  |
       |                  |                   |                 |
       |     B5           |                   |                 |

Bhushan, et al. [Page 6] RFC 171 THE DATA TRANSFER PROTOCOL June 1971

       The following error codes are assigned:
           Error Code            Meaning
       Hex           Octal
       00            000         Undefined error
       01            001         Out of sync. (type code other
                                 than B0 through BF).
       02            002         Broken sequence (the sequence #
                                 field contains the first expected
                                 but not received sequence number).
       03            003         Illegal DLE sequence (other than
                                 DLE DLE or DLE ETX).
       B0            260
    through       through        The transaction type (indicated by
       BF            277         by error code) is not implemented.
       The error code transaction is defined only for the purpose of
       error control.  DTP does not require the receiver of an error
       code to take any recovery action.  The receiver may discard the
       error code transaction.  In addition, DTP does not require that
       sequence numbers be remembered or transmitted.
 2B.7  Type B6 (abort) transactions have a fixed length of 2 bytes, as
       shown below.  First byte defines transaction type as B6, and
       second byte defines the abort function.
       |    Type           |    Function        |
       |                   |            | | |R| |
       |                   |            | |G|E| |
       |                   |            |F|R|C|U|
       |                   |            |I|O|O|N|
       |                   |            |L|U|R|I|
       |                   |            |E|P|D|T|

Bhushan, et al. [Page 7] RFC 171 THE DATA TRANSFER PROTOCOL June 1971

       The following abort codes are assigned:
            Abort Code                              Meaning
          Hex            Octal
          00             000              Abort preceding transaction
          01             001              Abort preceding unit
          02             002              Abort preceding record
          07             007              Abort preceding group
          0F             017              Abort preceding file
       DTP does not require the receiver of an abort to take specific
       action, therefore sender should not necessarily make any
       assumptions.  The manner in which abort is handled is to be
       specified by higher-level applications protocols.
 2B.8  Type B7 (NoOp) transactions are one byte long, and indicate no
       operation.  These may be useful as fillers when byte size used
       for network connections is other than 8-bits.

3. Initial Connection, Handshake and Error Recovery

 3A.  DTP does not specify the mechanism used in establishing
      connections.  It is up to the applications protocol (e.g., file
      transfer protocol) to choose the mechanism which suits its
      requirements. [6]
 3B.  The first transaction after connection is made will be type B3
      (modes available).  In a full-duplex connection, both server and
      user will communicate type B3 transactions, indicating modes
      available for send and receive.  In a simplex connection only
      sender will communicate a type B3 transaction.  It is the
      sender's responsibility to choose a mode acceptable to the
      receiver.  If an acceptable mode is not available or if mode
      chosen is not acceptable, the connection may be closed. [7]
 3C. No error recovery mechanisms are specified by DTP.  The
      applications protocol may implement error recovery and further
      error control mechanisms.


[1] The term transaction is used here to mean a block of data defined

    by the transfer mode.

[2] What constitutes a workable subset is entirely governed by the

    high-level application protocol.

Bhushan, et al. [Page 8] RFC 171 THE DATA TRANSFER PROTOCOL June 1971

[3] Transactions suppress the notion of host-IMP messages, and may have

    a logical interpretation similar to that of flags (and data)
    defined by Mealy in RFC 91.

[4] This assignment is made to be consistent with the TELNET philosophy

    of maintaining the integrity of the 128 Network ASCII characters.

[5] A 72-b9t descriptor field provides a convenient separation of

    information bits, as 72 is the least common multiple of 8 and 36,
    the commonly encountered byte sizes on ARPA network host

[6] It is, however, recommended that the standard initial connection

    protocol be adopted where feasible.

[7] It is recommended that when more than one mode is available, the

    sender should choose 'descriptor and count' mode (Type B2 or BA).
    The 'bitstream' mode (type B0 or B8) should be chosen only when
    the other two modes cannot be used.
        [ This RFC was put into machine readable form for entry ]
          [ into the online RFC archives by Samuel Etler 08/99 ]

Bhushan, et al. [Page 9]

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