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rfc:rfc264

Network Working Group A. Bhushan Request for Comments: 264 MIT NIC: 7812 B. Braden

                                                                  UCLA
                                                           W. Crowther
                                                                   BBN
                                                            E. Harslem
                                                            J. Heafner
                                                                  Rand
                                                           A. McKenzie
                                                                   BBN
                                                             J. Melvin
                                                                   SRI
                                                           B. Sundberg
                                                               Harvard
                                                             D. Watson
                                                                   SRI
                                                              J. White
                                                                  UCSB
                                                      15 November 1971
                     THE DATA TRANSFER PROTOCOL
 This paper is a revision of RFC 171, NIC 6793.  The changes to RFC
 171 are given below.  The protocol is then restated for your
 convenience.

CHANGES TO RFC 171

 1) The sequence number field is changed to 16 bits in the error (Type
    B5) transactions, thus resolving the ambiguity in the previous
    specification.  In addition, the information separators (Type B4)
    transactions shall also contain a 16-bit sequence number field.
 2) The modes available (Type B3) transactions shall define only the
    modes available for receive, instead of both receive and send.  In
    simplex connections modes available transactions should not be
    sent as they are meaningless.  In full-duplex connections, the
    modes available transactions are still required.
 3) The code assignments for "End Code" in information separators and
    for "function" in abort transactions have been changed to reflect
    a numerical order rather than "bit-coding".
 4) Minor editorial changes.

Bhushan, et. al. [Page 1] RFC 264 The Data Transfer Protocol 15 November 1971

I. INTRODUCTION

    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 may
    provide flexibility in implementation, and reduce complexity.
    Separating the data transfer function from the specific
    applications functions may also reduce proliferation of programs
    and protocols.
    We have therefore defined a data transfer protocol (DTP) which
    should be used for transfer of data in file transfer, remote job
    entry, and other applications protocols.  This paper concerns
    itself only with the data transfer protocol.  A companion paper
    (RFC 265) describes the file transfer protocol.

II. DISCUSSION

    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.

Transfer Modes

    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) convention.  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

Bhushan, et. al. [Page 2] RFC 264 The Data Transfer Protocol 15 November 1971

    "transparent" block does not begin with DLE STX, but with a
    transaction type byte.  This mode would 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 (i.e., not
    information) bits.  If a transaction has no filler bits, its
    filler count is zero.  This mode would be useful in most host-to-
    host data transfer applications.
    DTP allows for transfer modes to be intermixed over the same
    connection (i.e., the transfer mode is not associated with
    connection, but only with transaction).  The transfer modes can
    represent transfer of either data or control information.  The
    protocol allows for separating data and 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 subset of transfer modes is specifically
    permitted by DTP.  To provide compatibility between hosts using
    different subsets of transfer modes, an initial "handshake"
    procedure may be used.  The handshake involves exchanging
    information on modes available for receive.  This will enable host
    programs to agree on transfer modes acceptable for a connection.

Using DTP

    The manner in which DTP is used would depend largely on the
    applications protocol.  It is the applications protocol which
    defines the use of transfer modes and the use of information
    separator and abort functions provided in DTP (see
    SPECIFICATIONS).  For example, in a remote job entry protocol,
    aborts may be used to stop the execution of a job, while they may
    not 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 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

Bhushan, et. al. [Page 3] RFC 264 The Data Transfer Protocol 15 November 1971

    applications programs.  Another implementation may employ macros
    or UUO's (unimplemented user 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.

III. SPECIFICATIONS

 1.    Byte Size for Network Connection
       The standard byte size for network connections using DTP is 8
       bits.  However, other byte sizes specified by applications
       protocols 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 a connection is
       a sequence of transactions.  DTP defines the rules for
       delimiting transactions.
 2A.   Types
       The first 8-bit 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 will be referred to by the hexadecimal code
       assigned to them.  (The transaction types are discussed in more
       detail in Section 2B.)
          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.
       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
       through through         Unassigned but reserved for DTP.
       BF      277

Bhushan, et. al. [Page 4] RFC 264 The Data Transfer Protocol 15 November 1971

 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 the closing of a connection in bitstream
       mode is an implicit file separator (see Section 2B.5).
 2B.2  Type B1 and B9 (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 be 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).  [2] ETX is Hex '03' or
       Octal '03' (the same as ASCII ETX).
 2B.3  Type B2 and BA (descriptor and counts modes) transactions have
       three fields, a 9-byte (72-bit) descriptor field (as shown
       below) and variable length (including zero) info and filler
       fields.  The total length of a transaction is (72+info+filler)
       bits.
|<B2 or BA>|<Info count>| <NUL> <Sequence #>| <NUL> |<filler count>|
|<-8-bit-> |<--24-bit-->|<8-bit><--16-bit-->|<8-bit>|<---8-bit---->|
|<--------------------72-bit descriptor field--------------------->|
       _Info count_ is a binary count of the number of bits in the
       info field, not including descriptor or filler bits.  The
       number of info bits is limited to (2**24 - 1), as there are 24
       bits in info count field.
       _Sequence #_ is a sequential count in round-robin manner of B2,
       BA, and B4 type transactions.  The inclusion of sequence
       numbers will 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, all 1's, and zero for the
       first transaction only.
       _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.

Bhushan, et. al. [Page 5] RFC 264 The Data Transfer Protocol 15 November 1971

       The NUL bytes must contain all 0's.
 2B.4  Type B3 (modes available) transactions have a fixed length of
       two bytes, as shown below.  First byte defines the transaction
       type B3, and second byte defines the transfer modes available
       for receive.
       +-----------------+---------------------+
       |Type             |     I receive       |
       |        B3       |                     |
       |                 |0|0|BA|B2|B9|B1|B8|B0|
       +-----------------+---------------------+
       The modes are indicated by bit-coding, as shown above.  The
       particular bits, if set to logical "1", indicate that the
       corresponding modes are handled by the sender's receive side.
       The two most significant bits should be set to logical "0".
       Mode available transactions have no significance in a simplex
       connection.  The use of type B3 transactions is discussed in
       section 3B.
 2B.5  Type B4 (information separator) transactions have a fixed
       length of four bytes, as shown below.  First byte defines the
       transaction type B4, second byte defines the separator, and
       third and fourth bytes contain a 16-bit sequence number.
       +------------+------------+-------------------------+
       |Type        |  End Code  |      Sequence Number    |
       |     B4     |            |            |            |
       |            |            |            |            |
       +------------+------------+------------+------------+
       The following separator codes are assigned:
             Code                      Meaning
       Hex             Octal
       01              001             Unit separator
       02              002             Record separator
       03              003             Group separator
       04              004             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.

Bhushan, et. al. [Page 6] RFC 264 The Data Transfer Protocol 15 November 1971

       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 four
       bytes, as shown below.  First byte defines the transaction type
       B5, second byte indicates an error code, and third and fourth
       bytes may indicate the sequence number of a transaction in
       which an error occurred.
       +------------+------------+-------------------------+
       |Type        |  End Code  |      Sequence Number    |
       |     B5     |            |            |            |
       |            |            |            |            |
       +------------+------------+------------+------------+
       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 DLF sequence (other than DLE
                                 DLE or DLE FTX).
       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 two bytes,
       as shown below.  First byte defines the transaction type B6,
       and second byte defines the abort function.
       +------------+------------+
       |Type        |  Function  |
       |     B6     |            |
       |            |            |
       +------------+------------+

Bhushan, et. al. [Page 7] RFC 264 The Data Transfer Protocol 15 November 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
       03              003             Abort preceding group
       04              004             Abort preceding file
       DTP does not require the receiver of an abort to take specific
       action, therefore a sender should not make any assumptions
       thereof.  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 (8-bit) long, and
       indicate no operation.  These may be useful as fillers when the
       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. [3]
 3B.   The first transaction after a full-duplex connection is made
       will be type B3 (modes available) indicating the transfer modes
       available for receive.  The modes available (Type B3)
       transaction is not applicable in simplex connections.  It is
       the sender's responsibility to choose a mode acceptable to the
       receiver. [4]  If an acceptable mode is not available or if
       mode chosen is not acceptable, the connection may be closed.
 3C.   No error recovery mechanisms are specified by DTP.  The
       applications protocol may implement error recovery and further
       error control mechanisms.

Bhushan, et. al. [Page 8] RFC 264 The Data Transfer Protocol 15 November 1971

Endnotes

 [1]  The term transaction is used here to mean a block of data
 defined by the transfer mode.
 [2]  This assignment was made to be consistent with the TELNET
 philosophy of maintaining the integrity of the 128 Network ASCII
 characters.
 [3]  It is, however, recommended that the standard Initial Connection
 Protocol as specified in RFC 165 or any subsequent standard document
 be adopted where feasible.
 [4]  It is suggested that when available, the sender should choose
 'descriptor and count' mode (Type B2 or BA).  The 'indefinite
 bitstream' mode (Type B0 or B8) should be chosen only when the other
 two modes are not available.
       [ This RFC was put into machine readable form for entry ]
          [ into the online RFC archives by Ryan Kato 6/01 ]

Bhushan, et. al. [Page 9]

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