RFC 171
The Data Transfer Protocol
THE DATA TRANSFER PROTOCOL 💖
Okay bestie, first the title-block vibe check 🎀 This is RFC 171, filed under NIC 6793 in the Network Working Group, and it's dated 23 June 1971. Its categories are D.4, D.5, and D.7 🗂️. It updates 114 and obsoletes None, so nothing gets retired here. The squad behind it is Abhay Bhushan (MIT), Bob Braden (UCLA), Will Crowther and Alex McKenzie (BBN), Eric Harslem and John Heafner (Rand), John Melvin and Dick Watson (SRI), Bob Sundberg (HARVARD), and Jim White (UCSB). Main character energy, all of them 👑
I. INTRODUCTION ✨
Okay so there's one common protocol that basically everyone needs for moving data around 🦋 Think about it: remote job entry, file transfer, network mail, graphics, remote program execution, and talking to block data terminals (printers, card, paper tape, and magnetic tape equipment, especially with terminal IMPs). That's a LOT of different vibes.
We could cram every single one of those into one giant all-inclusive file transfer protocol 💅 but honestly? Splitting data transfer away from the application stuff gives implementers more flexibility AND keeps things simpler. It also means fewer programs and protocols getting spawned everywhere. Iconic 🧚
So we defined a low-level data transfer protocol (DTP) for moving data in file transfer, remote job entry, and other application protocols 🌸 This paper is ONLY about DTP, no distractions. The companion paper, RFC 172, is the one that describes file transfer protocol, so go check that out too 💜
II. DISCUSSION 🍓
The data transfer protocol (DTP) has three basic jobs ✨ First, it neatly chops NCP messages into "logical" blocks (transactions, units, records, groups, and files). Second, it lets you separate data from control information. Third, it includes some error control mechanisms. Not bad for a little protocol 💖
Three modes of separating messages into transactions [1] are allowed by DTP. Mode one is an indefinite bit stream that only ends when the connection is closed, so the whole bit stream is one single transaction for the entire life of the connection 🌊 This would be useful for data transfer between hosts and terminal IMPs (TIPs).
Mode two is the "transparent" block convention, which is kinda like the ASCII DLE (Data Link Escape) 🙈 In transparent mode, transactions (which MAY be arbitrarily long) end whenever the character sequence DLE ETX shows up (DLE and ETX are 8-bit character codes). To stop a DLE ETX from sneaking into the data stream, every DLE gets replaced with DLE DLE on the way out, and the extra DLE gets stripped on the way in. The plot twist 🫶: a transparent block starts with a transaction type byte instead of DLE STX like ASCII does. This mode will be useful for data transfer between terminal IMPs.
Mode three uses a count mechanism 🍓 Each transaction begins with a fixed-length descriptor field holding 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 lets you mix these modes on the same connection, because the mode belongs to the transaction and not the connection 💫 (i.e., mode is not associated with connection, but only with transaction). The transfer modes above can carry either data or control information. The protocol also lets you separate data from control at a lower level, using different "type" codes (see SPECIFICATIONS) for data and control transactions. This provision MAY simplify some implementations, periodt ✨
The implementation of a workable [2] subset of the above modes is specifically permitted by DTP, so you're totally allowed to only build some of them 💅 To keep hosts with different subsets compatible, an initial "handshake" procedure is REQUIRED by DTP. The handshake swaps information about which modes are available for transmit and receive, so host programs can agree on transfer modes that work for a connection 🤝
How DTP gets used really depends on the applications protocol, because that's the thing that defines the workable subset of transfer modes 👑 Like, the file transfer protocol won't work with ONLY the indefinite bit stream modes, bestie. For control information, at least one of the other two modes is REQUIRED. The use of the information separator and abort functions DTP provides (see SPECIFICATIONS) is also decided by the applications protocol. For example, in a remote job entry protocol, aborts MAY be used to stop a job from running, but in another applications protocol they MAY not cause any action at all 🙃
It SHOULD also be noted, bestie, that DTP does not define a data transfer service 🙅 There's no standard server socket and no initial connection protocol defined for DTP. What DTP does define is a mechanism for data transfer that can power block data transfers, file transfers, remote job entry, network mail, and tons of other applications 💖
There are to be no restrictions on how DTP gets implemented at different sites 🧚 For example, DTP MAY be embedded in an applications program (like file transfer), or it MAY be a separate service program or subroutine that several applications programs use. Another implementation MAY employ macros or UUO's (user unimplemented operations on PDP-10's) to achieve the functions specified in DTP. It's also possible that the separation between DTP and the applications protocols only exists at a conceptual level, iykyk 🌈
III. SPECIFICATIONS 🎀
1. Byte Size for Network Connection 💜
The standard byte size for network connections using DTP is 8-bit. Other byte sizes specified by higher-level applications protocols or applications programs are also allowed by DTP ✨ For this document, bytes are assumed to be 8-bits unless otherwise stated.
2. Transactions 🦋
At the DTP level, everything sent over a connection is a sequence of transactions. DTP defines the rules for delimiting those transactions [3].
2A. Types 🌸
The first byte of each transaction SHALL define a transaction type, as shown below 💅 (Note that code assignments do not conflict with the assignments in TELNET protocol.) The transaction types MAY be referred to by the hexadecimal code assigned to them. They're 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 (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 🌊
These transactions terminate only when the NCP connection is "closed". There's no other escape convention defined in DTP at this level. It SHOULD be noted that closing the connection in bitstream mode represents an implicit file separator (see section 2B.5).
2B.2 Type B1 and B0 (transparent block modes) transactions 🫧
These transactions terminate when the byte sequence DLE ETX is encountered. The sender SHALL replace any occurrence of DLE in the data stream with 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) [4].
2B.3 Type B2 and BA (descriptor and counts modes) transactions 📦
These transactions have three fields: a 9-byte (72-bits) descriptor field [5], plus variable length (including zero) info and filler fields, as shown below. The total length of a transaction is (72+info+filler) bits.
|<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|okay so info count is a binary count of how many bits are in the info field 💖 It doesn't count the descriptor or filler bits, so don't mix those up bestie 🧚 Since there are 24 bits in the info count field, the number of info bits is capped at (2**24 - 1) ✨
sequence # is a sequential count in a round-robin way for B2 and BA type transactions 🔁 Sequence numbers are lowkey giving debugging and error control energy 🕵️ They can be used to check for missing transactions and help track down errors. Hosts who aren't into this mechanism should have all 1's in the field 🙈 The count shall start from zero and go up sequentially to all 1's, and after that it resets to all zeros. The permitted sequence numbers are one greater than the previous one, and all 1's 💅
filler count is a binary count of the bits used as fillers (aka not actual information) after the end of the meaningful data 🫶 The number of filler bits is capped at 255, since there are 8 bits in the filler count field 🌸
The NUL bytes contain all 0's 🍓
2B.4. Type B3 (modes available) transactions 🎀
Type B3 transactions are a fixed 3 bytes long, as shown below 🦋 The first byte is the transaction type (B3), the second byte lists the modes available for send, and the third byte lists the 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 shown with bit-coding, as you can see above 🔥 If a particular bit (or bits) is set to logical "1", that mode is available. The 2 most significant bits should be set to logical "0" 💜 Type B3 transactions get discussed in section 3B, so peek there later 👑
2B.5. Type B4 (information separator) transactions 🌈
Type B4 transactions are a fixed 2 bytes long 🧚 The first byte is the transaction type (B4), and the second byte is 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|
+------------------+------------------+These separator codes are assigned:
Code Meaning
Hex Octal
01 001 Unit separator
03 003 Record separator
07 007 Group separator
0F 017 File separatorFiles, groups, records, and units can be data blocks that a user defines to be that thing 🎀 The only restriction is the hierarchy File>Groups>Records>Units (where '>' means 'contains') 💅 So a file separator doesn't just mark the end of a file, it also marks the end of the group, record, and unit, which is so iconic 😭 These separators may give you a handy "logical" split of data at the data transfer level 🌸 How they get used is governed by the applications protocol 🫶
2B.6. Type B5 (error codes) transactions 💀
Type B5 transactions are a fixed 3 bytes long 🙈 The first byte is the transaction type (B5), the second byte is an error code, and the third byte may indicate the sequence number where the error happened ✨
+------------------+-------------------+-----------------+
| Type | Error Code | Sequence # |
| | | |
| B5 | | |
+------------------+-------------------+-----------------+These 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 DLE sequence (other than
DLE DLE or DLE ETX).
B0 260
through through The transaction type (indicated by
BF 277 by error code) is not implemented.The error code transaction is only defined for error control, so no drama 🦋 DTP does not require the receiver of an error code to take any recovery action, and the receiver may discard the error code transaction 🥹 DTP also does not require that sequence numbers be remembered or transmitted 💜
2B.7. Type B6 (abort) transactions 🚫
Type B6 transactions are a fixed 2 bytes long, as shown below ✨ The first byte is the transaction type (B6), and the 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|
+-------------------+--------------------+These abort codes are assigned:
Abort Code Meaning
Hex Octal
00 000 Abort preceding transaction
01 001 Abort preceding unit
02 002 Abort preceding record
07 007 Abort preceding group
0F 017 Abort preceding fileDTP does not require the receiver of an abort to take any specific action, so the sender should not necessarily make any assumptions 🙃 How abort gets handled is up to the higher-level applications protocols 🌈
2B.8. Type B7 (NoOp) transactions 🧸
Type B7 transactions are one byte long and indicate no operation 💤 These may be useful as fillers when the byte size used for network connections is something other than 8 bits ✨
3. Initial Connection, Handshake and Error Recovery 🤝
3A. DTP doesn't specify the mechanism used to set up connections 🫶 It's up to the applications protocol (for example, a file transfer protocol) to pick the mechanism that fits its needs [6] 💅
3B. The first transaction after a connection is made will be type B3 (modes available) 🎀 On a full-duplex connection, both the server and the user send type B3 transactions to show the modes they have available for send and receive 🌸 On a simplex connection, only the sender sends a type B3 transaction 🦋 It's the sender's job to choose a mode the receiver can accept 👑 If there's no acceptable mode available, or the mode chosen isn't acceptable, the connection may be closed [7] 😭
3C. DTP doesn't specify any error recovery mechanisms 🙈 The applications protocol may implement error recovery and further error control mechanisms on its own 💖
END NOTES 📝
[1] The term transaction is used here to mean a block of data defined by the transfer mode 💜
[2] What counts as a workable subset is totally governed by the high-level application protocol 🧚
[3] Transactions skip the idea of host-IMP messages, and they can have a logical interpretation similar to flags (and data) defined by Mealy in RFC 91 ✨
[4] This assignment was made to match the TELNET philosophy of keeping the integrity of the 128 Network ASCII characters 🌈
[5] A 72-bit descriptor field gives a handy separation of information bits, since 72 is the least common multiple of 8 and 36, which are the commonly encountered byte sizes on ARPA network host computers 🍓
[6] It is, however, recommended that the standard initial connection protocol be adopted where feasible 🫶
[7] 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 ]
📦 where this page comes from
- ietf: rfc171 1971-06
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms