Network Working Group A. Bhushan
Request for Comments: 264 MIT
NIC: 7812 B. Braden
E. HarslemJ. Heafner
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
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.
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
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.
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.
Three modes of separating messages into transactions  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
"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.
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
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.
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
At DTP level, all information transmitted over a connection is
a sequence of transactions. DTP defines the rules for
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
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.
through through Unassigned but reserved for DTP.
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).  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)
|<B2 or BA>|<Info count>| <NUL> <Sequence #>| <NUL> |<filler count>|
|<--------------------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
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
|Type | I receive |
| B3 | |
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
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:
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.
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
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).
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 | |
| | |
The following abort codes are assigned:
Abort Code Meaning
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
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.  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.
 The term transaction is used here to mean a block of data
defined by the transfer mode.
 This assignment was made to be consistent with the TELNET
philosophy of maintaining the integrity of the 128 Network ASCII
 It is, however, recommended that the standard Initial Connection
Protocol as specified in RFC 165 or any subsequent standard document
be adopted where feasible.
 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 ]
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