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 HarslemJohn Heafner
John MelvinDick Watson
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
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.
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). 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  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
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
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.
At DTP level, all information transmitted over connection is a
sequence of transactions. DTP defines the rules for delimiting
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
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 (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) .
2B.3 Type B2 and BA (descriptor and counts modes) transactions have
three fields, a 9-byte (72-bits) descriptor field  and
variable length (including zero) info and filler fields, as
shown below. The total length of a transaction is
|<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
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.
| 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:
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 | | |
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 DLE sequence (other than
DLE DLE or DLE ETX).
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|
The following abort codes are assigned:
Abort Code Meaning
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
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. 
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.
 What constitutes a workable subset is entirely governed by the
high-level application protocol.
 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.
 This assignment is made to be consistent with the TELNET philosophy
of maintaining the integrity of the 128 Network ASCII characters.
 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
 It is, however, recommended that the standard initial connection
protocol be adopted where feasible.
 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 ]