RFC 171 (rfc171) - Page 2 of 9

The Data Transfer Protocol

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.