RFC 171 (rfc171) - Page 2 of 9
The Data Transfer Protocol
Alternative Format: Original Text Document
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.