Summary

  • RFC 913 proposed a Simple File Transfer Protocol with three file types. For a 512-word page on a 36-bit TOPS-20 machine, its ASCII, Binary and Continuous examples transmit 2560, 2048 and 2304 eight-bit bytes respectively.
  • Binary mode takes four eight-bit bytes from each 36-bit word and leaves the last four bits out; Continuous packs across word boundaries and carries all 36 bits. The mode determines what the recipient gets, even when the transfer itself completes cleanly.

The file-transfer window says “done.” Excellent. Which file did it deliver? That was a less comfortable question in 1984, when machines could disagree about the width of a word. Mark Lottor's RFC 913 proposed a Simple File Transfer Protocol—an old SFTP on TCP port 115, with a different design and era from the later SSH tool that inherited the same initials. Its central bargain was modest: offer more than TFTP, demand less machinery than FTP, and run commands and file data over one TCP connection. Inside that neat arrangement sat a menu with consequences larger than the program's name: TYPE A, TYPE B, or TYPE C.

Take the RFC's TOPS-20 example. A machine word has 36 bits. A disk page has 512 such words, or 18,432 bits. If “transfer the page” were a complete instruction, every mode would produce the same bit sequence. The RFC shows why it is not. In ASCII mode, five seven-bit characters occupy 35 bits of each word. The final bit stays out of the transmitted text; the character codes travel in eight-bit units with the high bit clear. Local line endings become CRLF under the NETASCII rules. For the example page, five characters per word make 2,560 transmitted bytes.

That is a text representation, with text's conversions, not a claim to preserve all 18,432 stored bits.

Binary mode sounds like the option that would settle the argument. It does not. It takes four eight-bit bytes from each 36-bit word—32 bits—and leaves bits 32 through 35 behind. The RFC's 512-word page therefore becomes 2,048 transmitted bytes. Whether those four omitted bits held meaningful file data would depend on how the file was stored. The point is that the word “binary” alone did not promise to carry the complete 36-bit source word. It promised eight-bit units. A machine that writes in groups of eight and a machine that writes in groups of 36 can both be behaving normally while disagreeing about the remainder.

Lottor supplied a third answer, Continuous mode. Ignore the 36-bit word boundary and feed the entire bit sequence into eight-bit containers. After four whole bytes from the first word, the remaining four bits join the first four bits of the second word. The pattern continues. All 512 words amount to 18,432 bits, or 2,304 eight-bit bytes. On a machine whose word width is already a multiple of eight, the RFC says Binary and Continuous should work alike. On TOPS-20, they answer different questions: “Which eight-bit chunks can I take from each word?” versus “How do I preserve the page's complete stream of bits?”

The receiving system still needs to know what those bits mean as a file, but the transfer has not silently thrown four bits from every source word overboard.

The surrounding protocol made the choice operational. SFTP commands were four ASCII letters followed by a NUL; the default file type was Binary. Before file operations, the server could require a user name, account and password. The proposal included listing directories, changing directories, renaming and deletion alongside retrieval and storage. A user who accepted the default without asking how a TOPS-20 file was packed could receive a perfectly normal success response for a byte stream that omitted part of each word. The mistake would lie in representation, not in whether the connection stayed up long enough to say goodbye.

The RFC Editor record now marks this 1984 proposal Historic. It does not supply a census of installations, and the document's three page sizes are worked examples rather than evidence that a particular user's data was lost. Their arithmetic still has teeth. A file is a physical sequence under one machine's storage rules, a transmitted sequence under a protocol's type rules, and a usable object only when the recipient can reconstruct the intended meaning. Three numbers—2,560, 2,048, 2,304—made that invisible negotiation visible. The program could say “file received”; the bits would have the last word.